In a small food processor, pulse the onion and garlic a few times until they are finely diced.
In a heavy skillet, heat olive oil over medium heat.
Add the ground beef, onions and garlic. Cook for 3 - 5 minutes, browning the beef and breaking up any large pieces.
Add the red bell pepper to the food processor and pulse a few times to mince.
Add the bell pepper and remaining ingredients (except the spinach + buns) to the skillet. Mix to combine.
Simmer for 20 minutes until the sauce thickens.
Spoon the meat sauce over the buns and top with baby spinach.