Learn French: Des noix ? J'en ai haché pour le gâteau. - Walnuts? I chopped some for the cake.

Normally, a past participle agrees with a preceding direct object. However, there is a major exception: when the preceding direct object is the pronoun en, the past participle never agrees. It stays in its default masculine singular form, haché.

Would it be correct to say Je les ai hachées instead?

That would be grammatically correct, but it changes the meaning. Je les ai hachées means I chopped them (referring to specific walnuts you already established). Using en means I chopped some, which fits better when talking about an indefinite amount for a recipe. Notice that if you use les, the past participle hachées does agree!

How can we tell that noix is plural in the first sentence?

The plural article des tells us it is plural. The feminine noun noix (walnut) ends in an x even in its singular form (une noix), so its spelling does not change when you make it plural.

I thought en replaced phrases starting with de. Where is the de here?