To make the filling, bring a medium pot of water to boil and boil the nopalitos for 20 minutes. Drain and rinse thoroughly to remove any slime.
In a large frying pan over medium-high heat, add enough oil to coat the pan. Add the onion and sauté for about 3 minutes until translucent. Add chopped garlic and nopalitos. Cook for five minutes, stirring frequently. Remove from heat and reserve.
To make the sauce preheat the broiler with the rack positioned on the rung closest to the heat source. Lightly coat the tomatillos and serrano peppers with some olive oil and place on a foil-lined rimmed baking sheet. Broil until blackened, turning once halfway through charring, about 5 minutes. Using tongs, place the vegetables in a blender and puree. The consistency should be like salsa. Reserve.
Heat the vegetable oil in a medium saucepan over medium heat, add the shallot and cook until translucent, 2 to 3 minutes. Add thyme, salt, and pepper and cook until shallots start to brown, 1 minute.
Immediately add wine and reduce until almost gone, 2 minutes. Stir in cream, tomatillo puree, and sugar. Cook over medium-high heat, stirring often, about 5 minutes, then reduce heat and simmer until sauce thickly coats a metal spoon, 3 to 5 more minutes.
To make the enchiladas, preheat the oven at 350F. Heat 2 large frying pans over medium high heat.
Leave one pan dry and heat 1/4 inch of oil and heat until shimmering in the second. Working with 1 tortilla at a time, cook in dry pan, turning once, until softened, 10 to 20 seconds, then in hot oil, turning once, until puffy and softened but not crisp, 10 to 15 seconds. Arrange in a single layer on paper towels.
Place the nopal mixture and Monterey Jack cheese down the center of each tortilla. Roll closed and place seam down in a 9-by-13-inch baking dish (with no sauce on the bottom).
Pour sauce over enchiladas; top with half the queso fresco. Bake until cheese melts and sauce is bubbling, 15 to 20 minutes.
Garnish with remaining queso fresco and cilantro.