Bring a large pot of salted water to a boil.
While the water heats, melt the 4 Tablespoons butter in a very large skillet or Dutch oven over medium heat.
One hot, add the 1-2 Tablespoons chile crisp and 4 cloves garlic and saute for 3-4 minutes, until the garlic is lightly golden.
Reduce the heat to low. Add the 1 cup heavy cream and 1 5-ounce package baby spinach and stir until wilted. Keep warm over low heat.
Cook the 1 pound fettuccine until al dente according to the package directions. Use tongs to transfer the noodles to the cream mixture, reserving the pasta water.
Add the ½ cup parmesan cheese, 1 Tablespoon lemon juice and zest and toss until the noodles are coated with the sauce, adding pasta water, if needed, to loosen the sauce.
Divide among serving dishes and top with Parmesan and more chile crisp, if desired.
