Cook spaghetti according to package directions.

Shape ground beef into 4 oval patties, about 1/2-inch thick.

Heat a large skillet over medium-high heat.

Fry patties in the skillet until browned on both sides.

Season patties with salt.

Remove patties from the skillet and set aside.

Add sliced onion to the drippings in the pan and cook until tender.

Add chopped garlic, sliced mushrooms, and green pepper strips to the pan.

Cook until vegetables are tender-crisp.

Drain off any excess fat from the pan.

Stir in minced fresh oregano (or crumbled dry oregano) and spaghetti sauce into the vegetables.

Heat the sauce to boiling, then reduce heat and simmer for 1 minute.

Return the beef patties to the skillet with the sauce.

Top each patty with a slice of provolone, mozzarella, or fontina cheese.

Heat until the cheese is melted and the patties are heated through.

Drain the cooked spaghetti and place it on a serving plate.

Use a pancake turner to transfer the beef patties and sauce over the spaghetti.

Garnish with fresh oregano, if desired.

Serve immediately.