Preheat the oven to 375 degrees. Brush a standard 12-cup muffin tin with olive oil, including the top of the pan.
Heat the olive oil over medium heat in a large (12-inch) sauté pan. Add the leeks and sauté for 3 minutes, until tender. Add the prosciutto and sauté for 2 to 3 minutes, breaking it up with a fork. Add the spinach, tossing with tongs, then cover the pan and cook for 3 minutes, tossing once while it cooks, until the spinach is all wilted. Off the heat, stir in the basil and lemon juice and set aside for 5 minutes. Divide the mixture with tongs among the 12 muffin cups, leaving any liquid behind. Sprinkle the Fontina evenly on top.
In a 4-cup liquid measuring cup, beat the eggs, half-and-half, 1/2 teaspoon salt, and 1/4 teaspoon pepper together with a fork. Pour the egg mixture evenly over the filling in each cup, filling the muffin cups to the top. Sprinkle each frittata with the Parmesan cheese.
Bake the frittatas for 20 to 25 minutes, until puffed and lightly browned on top. If you insert 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.
"Make it Ahead" by Ina Garten © Clarkson Potter 2014. Provided courtesy of Ina Garten. All rights reserved.