Jul 01 - al/en/en el desayuno?

The sentence (given by a native speaker) is: "Me gusta aplastar papas y comerlas como una sopa al desayuno" (I like to mash potatoes and eat them like a soup in/at breakfast)

I have 2 questions:

1. what is the correct word for "mash" in Spanish (used in Latin America, especially in Chile)? I've asked 3 native speakers, and they each have given me a different word, such as "aplastar", "moler", "mezclar"

2. should it be "al desayuno" or "en desayuno" or "en el desayuno"? Again, disagreement amongst the three native speakers I have asked.

Thanks.