Guacamole Grilled Cheese Sandwich

ripe avocados

small onion, minced

garlic, minced

small jalapeño, stems and seeds removed, minced

cilantro leaves, finely chopped

fresh lime juice

coarse salt

freshly grated black pepper

Roma tomato, chopped

slices crusty white bread

slices Cheddar cheese

Butter, for buttering bread