Preheat the oven to 375ºF. Brush a 12-cup muffin tin with olive oil, including on top of the pan.
Heat 2 tablespoons of olive oil over medium heat in a large pan. Add the leeks and cook for 3 minutes, until tender. Add the prosciutto and cook for 2-3 minute, break it up with a fork if they stick together.
Add the spinach, toss, then cook until wilted (about 1-2 minutes).
Turn off the heat, add julienne fresh basil. Squeeze in lemon juice.
Toss everything together and set aside for about 5 minutes.
Beat eggs, half-and-half, salt and pepper in a large bowl. Pour the egg mixture into a measuring cup.
Divide the spinach mixture with tongs among the 12 muffin cups, leave the liquid behind.
Sprinkle fresh grated Gruyère cheese on top.
Pour the egg mixture over each cup.
Sprinkle each frittata with fresh grated Parmesan cheese.
Bake for 20-25 minutes until puffed and lightly browned on top. If you insert a knife or a toothpick in the middle, they should feel firm.
Cool for 5 minutes, remove with a small sharp knife onto a serving plate, and serve hot or warm.
Notes
Ina's recipe uses Fontina which is an Italian cow's milk cheese. If you can't find Fontina, you can use Gruyère or Gouda or Provolone cheese instead. Also, I found the longer I leave the spinach in the pan, the mixture gets more wet so only cook it for 1-2 minutes until it just starts wilted.
Recipe by Let's Cook Some Food at https://www.letscooksomefood.com/mini-italian-frittatas/