Cheeseburger Stuffed Zucchini Boats

olive oil

ground beef (like Sirloin)

small yellow onion, diced

coarse salt and fresh black pepper

fresh tomatoes, diced, seeds removed

dill pickle, diced

Sauce:

butter

all-purpose flour

milk

Dijon mustard

+ ½ cups Gouda cheese, shredded, divided

medium zucchini

green onions, snipped with kitchen shears, for serving