Remove roots and dark green blades from leeks. Slice leeks crosswise as thinly as possible. Separate leek slices into individual rings. Rinse to get rid of any dirt between layers. Drain well and wrap leeks in kitchen towel to dry.
Line plate with paper towel and place it near stove. Pour vegetable oil into 12-inch skillet and set on medium heat. Spread leeks into even layer across surface of skillet. Cook, stirring almost constantly to ensure even cooking, until leeks are golden brown, about 10 minutes. Using slotted spoon, scoop crispy leeks out of oil onto prepared plate; set aside. Discard oil and wipe pan clean with paper towel.
Line another plate with paper towel and place it next to stove. Add soppressata to previously used skillet and sauté over medium-high heat, stirring often, until browned on all sides and fat has been rendered out; about 8-10 minutes. Transfer soppressata to prepared plate; set aside.
Cook pasta according to package directions until al dente; drain, rinse with cold water, return to cooking pot, and set aside.
Preheat oven to 400˚F. Grease 8x10-inch baking dish with 2 tablespoons butter; set aside.
In 1-quart saucepan, melt remaining 3 tablespoons butter over medium heat. Whisk in flour, milk, salt and pepper. Cook over medium heat, whisking often, until thick and bubbly, about 2-3 minutes. Remove saucepan from heat and whisk in cheeses.
Add sauce to pasta, followed by soppressata and peas; stir. Transfer pasta mixture to prepared baking dish. Cover dish with aluminum foil and bake until pasta is heated through and sauce is bubbly around edges, about 25-30 minutes.
Remove baking dish from oven; remove and discard foil. Sprinkle crispy leeks over top of mac and cheese. Serve immediately.