Preheat the oven to 475 degrees F (245 degrees C). Lightly grease a casserole dish.

Bring a large pot of lightly salted water to a boil; add orzo and cook for exactly 6 minutes. Place spinach in a strainer. Pour orzo into the strainer over spinach.

Transfer orzo and spinach to a mixing bowl and stir in Greek seasoning; set aside.

Meanwhile, add oil to a heavy skillet, and heat over medium-high heat until shimmering. Add onion, garlic, and ground beef. Cook and stir ground beef until browned and crumbly, 5 to 7 minutes. Stir in 1/2 cup tomato sauce.

Add beef mixture to orzo mixture; add lemon juice and mint, and stir to combine. Taste; adjust lemon juice and mint to taste.

Cut bell peppers in half lengthwise; carefully remove stem, seeds, and ribs. Stuff peppers with beef orzo mixture. Spread remaining 1/2 cup tomato sauce in the bottom of the prepared casserole dish. Place each stuffed pepper into the dish; cover with foil.

Bake in the preheated oven until peppers are tender, about 40 minutes. Remove foil. Sprinkle pine nuts and feta cheese over peppers.