Step 1: Preheat your oven to 375°F (190°C) and line a 13×18 inch rimmed sheet pan with parchment paper, greasing the edges well.
Step 2: Sauté the diced red bell pepper in olive oil over medium heat until soft, then set aside to cool slightly.
Step 3: In a large bowl, whisk the eggs, heavy cream, salt, and pepper until very well combined and slightly frothy.
Step 4: Pour the egg mixture into the prepared sheet pan, ensuring it spreads evenly to all corners.
Step 5: Sprinkle the sautéed peppers, chopped spinach, and crumbled feta cheese evenly over the top of the egg mixture.
Step 6: Bake in the center of the oven for 15 to 18 minutes, or until the eggs are set and no longer jiggle in the middle.
Step 7: Remove from the oven and immediately sprinkle with the fresh chopped basil.
Step 8: Allow the frittata to rest for 5 minutes before using a pizza cutter or sharp knife to slice into 12 squares.