Pour the olive oil into a large skillet and heat over medium-high heat. Add the Italian sausages and sear for a couple of minutes on each side, until golden brown on the outside. Transfer the sausages to the Crock Pot.
Add the sliced onion to the same skillet. Cook over medium heat for 5–7 minutes, stirring occasionally, until softened. Add a little more olive oil if the pan is too dry.
Add the sautéed onions and sliced bell peppers to the Crock Pot with the sausages.
Pour in the crushed tomatoes and add the tomato paste, balsamic vinegar, fennel seeds, salt, black pepper, garlic powder, and Italian seasoning.
Cover and cook on LOW for 6 hours or HIGH for 3 hours, until the sausages are cooked through and the peppers and onions are tender.
Toast the buns. Place one sausage on each bun and pile the saucy onions and peppers over the top.
Serve hot and enjoy!