Preheat oven to 425°.
Slice bread to almost the bottom to make pockets. Being sure not to slice all the way through.
Heat olive oil over medium-high heat in a sauté pan and add Brussels sprouts. Toss to coat.
Sauté until golden on both sides. Add vinegar, salt & pepper. Toss & remove from heat.
Let cool a little and roughly chop. Set aside.
Melt butter in a pan. Add onions, garlic & salt. Toss & sauté until caramelized, 5-7 minutes. Remove from heat.
Stuff bread with onions first, then shredded cheese, then brussels sprouts.
Bake 10-12 minutes, until cheese has fully melted and everything is slightly golden on top.
Top with diced tomatoes & serve.