Preheat the oven to 375. Slice the leeks into rounds and saute over medium heat in 1 tabespoon of olive oil until starting to soften. Add the washed spinach one handful at a time and cover to wilt. Continue cooking until all the spinach is just wilted but not completely cooked through. Turn off the heat.
Meanwhile add the pasta to boiling water and cook per package instruction. While the pasta is cooking, heat the butter over medium heat until starting to foam. Add the flour and whisk continually until starting to take on color. After about 2 minutes slowly pour in the half and half and bring just to a simmer. Add the mustard and nutmeg. Simmer for 2 - 3 minutes then turn to low and add the grated cheeses (reserve 1/4 cup for topping). Use a spatula or spoon to fold the cheese into the cream mixture gently. Continue stirring until completely smooth. Taste and add salt and pepper as needed (this will depend on the saltiness of the cheese). Gently fold the spinach and leek mixture into the cheese sauce until evenly incorporated. Use a food processor to chop the slices of sourdough into bread crumbs. Add to a pan with 1 tablespoon of olive oil over medium high heat and toast until turning a deep golden color.
Drain the pasta and fold into the cheese sauce. Top with the remaining cheese and the bread crumbs. Bake at 375 for 30-45 minutes until golden and crispy on top.