In dat recept moeten de groenten langzaam garen.

Dutch follows a V2 (verb-second) rule for the finite verb and then pushes any non-finite verb (infinitive) to the end. Here:

1. In dat recept (adverbial)
2. moeten (finite verb, position 2)
3. de groenten langzaam (subject + adverb)
4. garen (infinitive, final position)

This ordering is mandatory in main clauses with auxiliaries or modals.

Both garen and koken relate to cooking, but:

• garen means “to cook through” or “to simmer/gently cook until done.” It focuses on the process of bringing food to the right doneness.
• koken usually means “to boil” or “to cook” in a more general or higher-temperature sense.

In recipes you might say “groenten garen in bouillon” rather than “groenten koken in bouillon” if you want a gentle simmer.

Adverbs of manner (like langzaam) typically come directly before the verb they modify:

… de groenten langzaam garen …

If you move langzaam elsewhere, you can still be understood, but it sounds less natural:

• “de groenten garen langzaam” (possible, but uncommon)
• “langzaam moeten de groenten garen in dat recept” ( grammatically OK, shifts emphasis to langzaam )

The default, most idiomatic order in a recipe is the one you saw.

Dutch uses in with recept when you mean “in a recipe” (as a set of instructions).

• op dat recept would suggest “on the printed page of the recipe.”
• bij dat recept can mean “together with” or “concerning that recipe,” but is less precise.
  You can also say volgens het recept (“according to the recipe”).

Recept is a het-word, so the corresponding demonstratives are dit (this) and dat (that).

• Use dit recept to point to something physically or conceptually very close (“in this recipe”).
• Use dat recept to refer back to a recipe already mentioned or somewhat further away.
  You never say deze recept because deze is for de-words.