Preheat your oven to 375°F (190°C).
Slice the top off the bell pepper and carefully remove the seeds and membranes. Set the pepper aside.
Finely chop the quarter onion, garlic clove, and half tomato.
Heat the olive oil in a pan over medium heat. Sauté the chopped onion and garlic until softened, about 2-3 minutes.
Add the lean ground turkey to the pan, breaking it apart. Cook until the turkey is browned and mostly cooked through, about 5-7 minutes.
Stir in the chopped tomato and fresh spinach. Season the mixture with salt and pepper, and let it cook for an additional 2 minutes until the spinach wilts.
Spoon the turkey and vegetable mixture into the hollowed bell pepper, pressing the filling down gently.
Place the stuffed bell pepper in a baking dish and cover loosely with foil.
Bake in the preheated oven for 20 minutes, then remove the foil and bake for an additional 5-10 minutes to allow the top to lightly brown.
Remove from the oven, let cool slightly, and serve warm.