Whisk the ground mustard, paprika, and garlic powder together in a small bowl until evenly combined. Mixing the seasonings beforehand helps disperse them uniformly through the pasta and reduces the chance of clumping once they meet the liquid.
Add the 4 cups of water, elbow macaroni, and fine sea salt to a large pressure cooker or suitable pot. Stir in the prepared seasoning blend until it is distributed throughout the pasta.
Add the unsalted butter, secure the lid, and bring the cooker to high pressure. Cook for 4 minutes.
When the cooking time is complete, carefully release the pressure following the instructions provided for your pressure cooker.
Pour the evaporated milk directly into the cooked macaroni and stir thoroughly. The heat remaining in the pasta will warm the milk and begin forming the creamy sauce base.
Add the freshly shredded mild cheddar gradually, about 1 cup at a time, stirring continuously after each addition until the cheese is fully melted before adding more.
Add the shredded Monterey Jack and continue stirring until it has completely melted into the mixture and the sauce becomes smooth, silky, and evenly combined.
Taste the macaroni and cheese and adjust the seasoning if necessary. Add additional salt if desired, along with freshly cracked black pepper to taste.
Serve immediately while the cheese sauce is warm and at its creamiest consistency.