Grate onion on large holes of a grater to get a packed 1/2 cup; dice bell pepper. Preheat oven to 400°F.
In a large pan, melt 1-1/2 tablespoons butter over medium-high heat. Add onion, bell pepper, and 2 teaspoons garlic. Sauté until tender and liquid has evaporated (about 5–7 minutes).
Push veggies to edge; add beef to center. Let sear, then crumble while incorporating veggies. When mostly browned through, add tomato paste, bouillon, pepper, ground mustard, and a pinch of salt. Drain excess grease if needed.
Add crushed tomatoes, ketchup, Worcestershire, and brown sugar. Bring to a simmer, then reduce heat to low and simmer until thick (10–15 minutes). Stir occasionally. Remove from heat. Taste and adjust seasonings.
Mix remaining 3-1/2 tablespoons butter with remaining 2 teaspoons garlic and 1/4 teaspoon salt. Cut French bread in half lengthwise. Then cut both halves in half widthwise to get 4 large pieces.
Brush a little bit of butter on the bottom of all pieces. Place upside down on a foil-lined sheet pan. Bake for 5 minutes. Remove and flip over. Brush rest of butter on inside of bread. Bake for 5 minutes. Remove and let stand, then gently press in the centers to make room for meat sauce.
Divide beef mixture evenly among bread. Top with cheese. Bake 10 minutes or until cheese melts.
Slice and enjoy immediately, garnished with parsley if desired.
Video
Notes
Note 1: Adjust brown sugar to taste based on the acidity of the tomatoes used. You might skip it or add a bit more. Taste and adjust accordingly.Storage: For leftovers, let cool to room temperature and store in an airtight container in the fridge for 2–3 days. Freeze slices on a sheet pan, then in a bag for up to 2 months.