PREP: Remove 1 cup of frozen peas from the freezer to begin thawing. Grate the cheese, measure, and set aside 2 cups to get to room temperature. Measure milk and set aside to get to room temperature.
ONION AND GARLIC: Heat the olive oil in a large non-stick skillet set to medium-high heat. Once the oil is shimmering, add in the onion. Sauté for 5-7 minutes or until golden and add in the garlic. Stir until fragrant, about 30 seconds.
COOK PASTA: Add in 1 cup of the milk, the water, broth, and UNCOOKED pasta shells. Add some salt and pepper to taste (I add 1 tsp fine sea salt and 1/2 tsp pepper). If desired, add in 1/2 teaspoon each of garlic and onion powder (optional, but nice). Still over medium-high heat, bring the mixture to a simmer (bubbling at the edges) and then lower the heat to medium and cook, uncovered, stirring often until the pasta is al dente (about 5-7 minutes). The pasta will continue to cook a bit over the next few minutes, so don’t overcook it here (Note 4).
ADD TUNA: Reduce heat to low. Stir in the remaining 2 tablespoons milk, Dijon mustard, and 1 tablespoon fresh lemon juice. Add the drained tuna and mix gently until combined. Season with salt and pepper. (I add another 1/4 up to 1/2 teaspoon of each.) Stir in the frozen peas. Remove pot from heat and let stand for 2 minutes.
CHEESE: Off heat, add freshly shredded cheese, and gently stir through, letting the residual heat melt the cheese. Let the pasta rest, stirring occasionally, until it thickens and gets nice and creamy. Taste once more, adding additional seasoning if needed. Garnish with fresh parsley if desired and enjoy immediately.