Heat oil in a large heavy skillet over medium. Add leeks, garlic, salt and pepper. Cook until jammy, stirring occasionally, about 10-15 minutes.
While leeks cook, prepare pasta according to package directions, making sure to save at least 1 cup of pasta water.
Add heavy cream and parmesan cheese to the leeks. Stir to combine.
Add cooked pasta, lemon juice, and ½ cup of reserved pasta water. Stir until pasta is coated in sauce. Add more pasta water to thin sauce as needed.
Taste, adjust seasoning, and serve.
