Learn French: Combien de kilos d'oignons est-ce que tu prends ? - How many kilos of onions are you taking?

Combien (how much / how many) is an expression of quantity. In French, expressions of quantity are always followed by a simple de (or d' before a vowel), never des—even when the noun following it is plural, like kilos.

Since "onions" is plural, why is it d'oignons instead of des oignons?

This follows the exact same rule as above! Words like kilo, litre, or beaucoup (a lot) also express a specific quantity. Because of this, "a kilo of onions" is always un kilo d'oignons, using a simple de (which drops its 'e' and becomes d' before the vowel 'o').

What does est-ce que do in this sentence?

Est-ce que doesn't have a direct English translation here. It acts as a question marker. By placing it after the question phrase (Combien de kilos d'oignons), you can keep the rest of the sentence in normal subject-verb order (tu prends) instead of having to flip them.

The English translation says "are you taking", but doesn't tu prends just mean "you take"?

French doesn't have a separate "-ing" tense (present continuous) like English does. The single French present tense (tu prends) covers both "you take" and "you are taking". We translate it as "are you taking" here simply because it sounds more natural in English.