Step 1
Heat a wide, deep 12-inch skillet or Dutch oven over medium-high. When hot, add the olive oil and leeks, reduce heat to medium and cook, stirring constantly, until softened and lightly golden, 5 minutes.
Step 2
Stir in the garlic and mushrooms. Cook, undisturbed, for 2 minutes, then stir and cook, undisturbed, until the mushrooms start to caramelize and stick to the bottom of the pan, another 2 to 3 minutes. (This allows the water from the mushrooms to evaporate, encouraging caramelization.) Add the juice of one lemon half to deglaze the pan and stir to combine.
Step 3
Add the pasta and stock; stir well. Cover with lid and cook on medium heat. Begin tasting pasta 10 minutes after you add it. (All pasta shapes and brands will have different cooking times. You want the pasta to be just tender.)
Step 4
Once the pasta is cooked, turn off the heat and add the butter and parsley. Squeeze in the juice from the remaining lemon half and stir until butter is melted and dispersed. Season with salt and pepper.
Step 5
To serve, top with ricotta, red-pepper flakes (to taste) and more chopped parsley.
