• Preheat oven to 375˚F and lightly grease a shallow 2 quart baking dish.
• Bring a large pot of water to a boil for the pasta. Add 2 teaspoons salt. When the water returns to a boil, stir in the pasta and cook until al dente, about 6 minutes. Drain the pasta and set aside.
• Meanwhile, in a medium sauté pan over medium heat, warm 2 tablespoons of the olive oil. Add the leeks and ½ teaspoon salt. Sauté for 5 to 7 minutes, until the leeks are lightly browned. Remove from the heat and set aside.
• Warm the remaining 3 tablespoons olive oil in a medium saucepan over medium-low heat and whisk in the flour. Cook for 3 to 4 minutes, whisking every 30 seconds, until the flour is a darker shade of brown.
• Raise the heat to medium, add the milk, and bring to a simmer, whisking constantly. Simmer for 10 minutes, whisking constantly, until thickened.
• Reduce the heat to low, add the vinegar, paprika, cayenne, ½ teaspoon salt, ½ cup of the cheddar, ½ cup of the pepper jack, the mozzarella, and Parmesan to the white sauce and stir until smooth. Season with black pepper to taste.
• Add the leeks and elbows to the white sauce, gently stir well, and pour into the baking dish.
• Sprinkle the remaining ½ cup cheddar and ½ cup pepper jack over the top and garnish with the chopped rosemary.
• Bake for 35 to 40 minutes, until lightly browned and crisp on top. Serve hot.
NOTE: CLEANING LEEKS
• First cut off the greens, leaving only the light green and white bottom, then cut off the roots. Slice the leek in half lengthwise, then thinly slice it horizontally. Fill a large bowl with cold water. Add the leek slices, and swish around well, vigorously, rubbing any dirt and sand off the pieces. Lift from the water and set in a colander. Rinse the bowl of any residue and repeat if necessary.