Sauté both in a bit of oil over medium heat until translucent. Then increase the heat and brown the ground beef.
Mix in the tomato paste and spices. Season with salt and pepper.
Halve and core the bell peppers. Divide the filling among the halves and wrap them in aluminum foil.
Grill the foil packets for about 10-15 minutes, or until the bell peppers are soft. Unwrap them, crumble some feta cheese over the top. Garnish them with some chopped parsley, if desired.