Preheat oven to 350 degrees F
Heat skillet over medium heat, when warm, lightly spray with cooking spray, then, wilt the spinach, add a pinch of salt, and light squeeze of lemon, set aside
Coat six muffin cups with cooking spray, set aside
Gently press prosciutto into each prepared muffin cup, carefully ruffling the edges to ensure they are not touching the cup next to them
Break an egg into a measuring cup (this helps to prevent any shells from being left behind), and gently slip the egg into a muffin cup
Using tongs, gently place wilted spinach on the side of the yolk, followed by 2 tomato halves
Top with generous pinch of salt, freshly cracked pepper, red pepper flakes (if desired)
Bake for 18-20 minutes, or until the whites are completely set, and yolks are thickened. Let stand in the muffin cups for 3-5 minutes before serving. Carefully remove egg cups from muffin cups, and sprinkle with lemon zest and additional salt and pepper, if desired