hakusai to moyasi ni goma wo kakeru dake de, kantanna sarada ga dekimasu.

Because に marks what the sesame is being put onto.

In this sentence:

• ごまをかける = sprinkle/add sesame
• 白菜ともやしに = onto napa cabbage and bean sprouts

So the structure is:

• 白菜ともやしに = onto the cabbage and bean sprouts
• ごまをかける = sprinkle sesame

The vegetables are the target, so they take に. The sesame is the thing being sprinkled, so it takes を.

Here と simply means and.

So:

• 白菜ともやし = cabbage and bean sprouts

It is just linking two nouns into one list.

Because ごま is the direct object of かける.

In other words, ごま is the thing being applied/sprinkled, so it takes を:

• ごまをかける = sprinkle sesame

A useful way to think about it is:

• に = where it goes
• を = what you put on

In this sentence, かける means something like:

• to sprinkle
• to pour over
• to put on top of

So ごまをかける means to sprinkle sesame on the vegetables.

This is a very common verb in food-related Japanese. Depending on the object, it can mean things like:

• ソースをかける = pour sauce on
• 塩をかける = sprinkle salt on

So this is not the hang meaning of かける.

だけ means only or just.

When you see verb + だけで, it often means:

• just by doing...
• with only...
• all you have to do is...

So:

• ごまをかけるだけで = just by sprinkling sesame
• just by adding sesame

The で here shows that this action is enough to produce the result in the next part of the sentence.

Because 簡単 is a na-adjective, not an i-adjective.

When a na-adjective comes directly before a noun, it takes な:

• 簡単なサラダ = a simple salad

Compare:

• 静か → 静かな部屋
• 有名 → 有名な店

But when it is at the end of a sentence, you do not use な:

• このサラダは簡単です = This salad is simple.

Because できる usually takes が, not を, when it means be made, be completed, or come into being.

So:

• サラダができます = a salad gets made / a salad is ready / you can make a salad

This is different from a transitive verb like 作る, where you would say:

• サラダを作る = make a salad

With できる, the salad is treated more as the thing that comes into existence, so が is natural.

It can feel like both in English, which is why this expression can be tricky.

In this sentence, できます means something like:

• you can make
• a salad is ready
• you end up with a simple salad

In recipe-style Japanese, できます often sounds less like talking about ability and more like talking about the result:

• do this, and a salad is made
• do this, and you have a salad

So here it is more natural to understand it as a result, not just pure ability.

Because Japanese often omits subjects when they are obvious from context.

In a recipe or cooking instruction, the implied subject is usually:

• you
• or a general someone doing this

So even though the sentence does not explicitly say you, the meaning is understood naturally.

This is very common in Japanese, especially in:

• recipes
• instructions
• casual conversation

The comma separates the setup from the result.

The first part:

• 白菜ともやしにごまをかけるだけで、
  = just by sprinkling sesame on cabbage and bean sprouts,

The second part:

• 簡単なサラダができます。
  = a simple salad is made / you get a simple salad.

So the comma works a bit like a pause before the result. It is not the same as an English grammar rule exactly, but it helps readability.

Yes. It works as one combined phrase meaning:

• onto the cabbage and bean sprouts

First, 白菜ともやし makes a noun list:

• 白菜と もやし = cabbage and bean sprouts

Then に applies to the whole list:

• 白菜ともやしに = onto cabbage and bean sprouts

So you should read it as one unit, not as if に belongs only to もやし.