Leg de tomaat op de snijplank en voeg een theelepel olie en een snufje zout toe.

They are imperative forms, used for giving commands or instructions. Recipes often use this style.

• leggen → leg
• toevoegen → voeg ... toe

Dutch often leaves out the subject in instructions, just like English does in Put the tomato... or Add....

The full verb is toevoegen, which is a separable verb. In many main clauses, including commands, Dutch splits this kind of verb into two parts:

• voeg = the conjugated verb part
• toe = the separable particle, placed later in the sentence

So you get:

• Voeg een theelepel olie toe.

But when the verb is in the infinitive, it stays together:

• Ik wil olie toevoegen.

That is normal for separable verbs in Dutch main clauses. The main verb part comes earlier, and the particle often goes to the end.

So:

• toevoegen → voeg ... toe
• aandoen → doe ... aan
• opeten → eet ... op

In your sentence, the whole object een theelepel olie en een snufje zout comes before the final toe.

Because Dutch usually writes compound nouns as a single word.

• snijplank = cutting board
• theelepel = teaspoon

This is very common in Dutch. English often writes these as two words, but Dutch usually combines them.

Also, snij- comes from the verb snijden. In compounds, verb forms often get shortened like this:

• snijden → snijplank
• slaapkamer from slapen
• wasmachine from wassen

Snufje means a pinch. The ending -je is a diminutive ending, which is extremely common in Dutch.

So:

• snuf = a sniff / small amount
• snufje = a little pinch / a small amount

Dutch uses diminutives a lot, especially for small quantities or in everyday speech. In cooking, een snufje zout is the normal way to say a pinch of salt.

A useful grammar point: diminutives are always het words, so it is het snufje.

This is the difference between definite and indefinite articles.

• de tomaat = the tomato
• de snijplank = the cutting board
• een theelepel = a teaspoon
• een snufje = a pinch

The tomato and the cutting board are treated as specific things in the situation, so Dutch uses de. The teaspoon and pinch are quantities being introduced, so Dutch uses een.

Also:

• tomaat is a de-word
• snijplank is a de-word
• snufje is a diminutive, so it is a het-word, though here you only see een, not het

After words of quantity or measurement, Dutch usually puts the substance directly after them, without van.

So Dutch says:

• een theelepel olie
• een glas water
• een kilo aardappelen
• een snufje zout

This works much like English a teaspoon of oil, but Dutch normally does not need a separate word corresponding to of here.

Because they are being used as mass nouns inside quantity expressions.

• een theelepel olie
• een snufje zout

When you say how much of something you add, Dutch usually does not use an article before the ingredient itself.

Compare:

• Ik koop olie. = I buy oil.
• Voeg een theelepel olie toe. = Add a teaspoon of oil.

You would only use an article if you meant a specific oil or salt in a more specific context, such as de olie or het zout.

Yes. It stays together when it is not being split by main-clause word order.

For example:

• Ik wil olie toevoegen.
• Het is belangrijk om zout toe te voegen.
• Omdat je olie toevoegt, wordt het zachter.

But in a main clause or command, it usually splits:

• Ik voeg olie toe.
• Voeg olie toe.

So the split form in your sentence is completely normal.

Yes. These are two coordinated commands joined by en.

Each clause follows normal imperative structure:

• Leg de tomaat op de snijplank
• en voeg een theelepel olie en een snufje zout toe

In both parts, the verb comes first because they are commands. In the second part, the separable particle toe goes to the end.

So the sentence structure is very typical for recipe instructions.