Slice off the tops of the tomatoes and scoop out seeds and pulp. Lightly salt the interior and set upside-down on paper towels to drain for 10 minutes.
In a skillet over medium heat, warm 1 tbsp olive oil. Sauté red onion until translucent, ~3 minutes. Add garlic and cook 30 seconds more.
Stir in chopped spinach and cook until just wilted. Season with oregano, salt, and pepper.
Add shrimp to the pan and cook 2–3 minutes per side until opaque. Remove from heat and roughly chop into bite-size pieces.
In a bowl, combine cooked brown rice, shrimp-spinach mixture, lemon juice, remaining olive oil, and feta (if using). Adjust seasoning.
Place tomato shells in a baking dish and fill each with the rice-shrimp mixture. Drizzle any remaining filling over the top.
Bake 15–18 minutes until tomatoes are tender but still hold shape.
Garnish with fresh basil leaves and serve warm.
Notes
Use day-old brown rice for best texture and lower glycemic impact.
Leftover filling can be wrapped in lettuce leaves for a quick lunch.