Hver gang jeg lager middag på tirsdager, hakker jeg sopp og spinat først.

Because hver gang already works as a time expression introducing the clause, similar to every time or whenever in English.

So Norwegian goes straight to:

• Hver gang jeg lager middag ...

not:

• Hver gang at jeg lager middag ...

In standard Norwegian, adding at here would be wrong.

Yes, often you could.

• Hver gang jeg lager middag på tirsdager, ...
• Når jeg lager middag på tirsdager, ...

The difference is mainly nuance:

• hver gang = every single time, a bit more explicit and emphatic
• når = when / whenever, more neutral

So hver gang stresses repetition a little more strongly.

This is because of Norwegian word order.

The sentence starts with a whole subordinate clause:

• Hver gang jeg lager middag på tirsdager

After that comes the main clause. In Norwegian main clauses, the finite verb normally comes in second position. That means the verb comes before the subject here:

• ..., hakker jeg sopp og spinat først.

This is a very important rule in Norwegian, often called the V2 rule.

Compare:

• Jeg hakker sopp og spinat først.
• På tirsdager hakker jeg sopp og spinat først.
• Hver gang jeg lager middag på tirsdager, hakker jeg sopp og spinat først.

In all of these, the finite verb is in the second slot of the main clause.

Because the sentence begins with a subordinate clause, and then the main clause follows.

• Hver gang jeg lager middag på tirsdager, = subordinate clause
• hakker jeg sopp og spinat først. = main clause

In Norwegian, it is standard to put a comma between them when the subordinate clause comes first.

Because på tirsdager means on Tuesdays / every Tuesday, so it describes a repeated habit.

• på tirsdag = on Tuesday, usually one specific Tuesday
• på tirsdager = on Tuesdays, regularly

Since the whole sentence describes something habitual, the plural fits naturally.

Because tirsdager is the indefinite plural, which is often used for general habits and repeated times.

• på tirsdager = on Tuesdays, in general
• på tirsdagene = on the Tuesdays, meaning some specific Tuesdays already known from context

So in a general routine statement, på tirsdager is the normal choice.

Yes, it is understandable and grammatical, but it is a little repetitive because both parts suggest repetition.

• hver gang already means every time
• på tirsdager also gives a habitual meaning: on Tuesdays

So the sentence is fine, but depending on context, a Norwegian speaker might also say:

• Hver gang jeg lager middag, hakker jeg sopp og spinat først.
• Når jeg lager middag på tirsdager, hakker jeg sopp og spinat først.

The original sentence is still perfectly usable; it just emphasizes that this repeated action happens specifically on Tuesdays.

Yes, lage literally means make, but in Norwegian lage middag is the normal way to say make dinner or cook dinner.

So:

• lage middag = make/cook dinner

This is very idiomatic Norwegian.

Also, middag in modern Norwegian usually means the main cooked meal of the day, often what English speakers would call dinner.

Because meal words are often used without an article when speaking generally.

So Norwegian says:

• lage middag = make dinner
• spise frokost = eat breakfast
• lage lunsj = make lunch

Using an article would usually make it more specific, and in this sentence the general expression lage middag is the natural one.

Here sopp is being used in a general food-ingredient sense, similar to English mushroom or mushrooms when talking about cooking.

In Norwegian, ingredient words are often used in a more mass-like or general way:

• hakke sopp = chop mushrooms
• kjøpe tomat / kjøpe tomater can vary depending on style and meaning
• tilsette løk = add onion

If you wanted to focus on countable individual mushrooms, sopper could be possible in another context. But in cooking language, sopp is very natural.

Because they are being mentioned as ingredients in a general sense.

Norwegian often leaves out articles with food ingredients when talking about what someone cooks, chops, buys, adds, and so on:

• hakke sopp og spinat
• kjøpe melk
• tilsette salt
• steke løk

This is similar to English in some cases, but Norwegian often does it even where English might prefer a plural noun such as mushrooms.

Because Norwegian often uses the present tense for habits and repeated actions, just like English does.

The sentence describes a routine:

• Hver gang jeg lager middag på tirsdager ...
• ... hakker jeg sopp og spinat først.

This is the normal tense choice for something that happens regularly.

Because that is a very natural place for it in Norwegian.

• hakker jeg sopp og spinat først

Here først means first and tells us the chopping happens before other actions. Putting it at the end sounds smooth and neutral.

It can sometimes move for emphasis, for example:

• ... hakker jeg først sopp og spinat.

But the original version is very natural and probably the most straightforward choice here.