Make the dough: Whisk the semolina and all-purpose flour in a large bowl. Mound on a work surface and make a well in the center. Pour 1/2 cup cold water into the well. Using a fork and working your way around the well, gradually mix the flour into the water. Continue mixing until the dough is crumbly. If the dough is too dry to come together, add up to 1/4 cup more water, a little at a time.
Knead the dough until smooth and elastic, about 6 minutes. Wrap in plastic wrap and let rest at room temperature,30 minutes. (The dough can be made a day ahead and refrigerated; bring it to room temperature before rolling.)
Divide the dough into 4 pieces. Working with 1 piece at a time (and keeping the other pieces wrapped), roll the dough on the counter with your hands into a 1/2-inch-thick log. With a knife or bench scraper, cut the log into 1/2-inch pieces; roll each piece into a small ball. With floured hands, press your thumb into a dough ball and drag it toward you to form a bowl shape; put the dough on the tip of your index finger and turn it inside out. Repeat to form the remaining orecchiette; transfer to a floured baking sheet as you go.
Bring a large pot of salted water to a boil. Meanwhile, melt the butter with the olive oil in a large skillet over medium heat. Add the leeks and cook, stirring occasionally, until softened but not browned, 7 to 8 minutes. Add the ham and cook until heated through, about 3 minutes. Stir in the garlic and thyme and cook1 minute. Add the peas and broth and bring to a simmer. Stir in the heavy cream and lemon juice, season with salt and pepper and simmer until the peas are tender, about 5 minutes for frozen, 10 minutes for fresh.
Transfer the orecchiette to a colander and shake over the baking sheet to remove the excess flour. Add the orecchiette to the boiling water and cook until al dente, about 8 minutes. Remove with a strainer or slotted spoon and add directly to the skillet with the ham and peas. Toss, adding spoonfuls of the pasta cooking water to loosen, if necessary. Remove from the heat and stir in the parmesan.
Fresh pasta can be frozen for up to 2 months. Freeze on baking sheets, then transfer to zip-top bags. Cook from frozen, about 2 extra minutes.