In a medium mixing bowl, combine the mustards, maple syrup, cider vinegar, garlic, 2 tablespoons olive oil, and 1/4 teaspoon salt.
Line a rimmed baking sheet with parchment paper. Arrange the pork tenderloin in the center and spoon half the mustard sauce over the top. Using a spatula (or your hands!), slather it over the pork and make sure it’s in every nook and cranny. Set aside at room temperature.
Half the leeks lengthwise and rinse them under cold water, fanning out the layers to make sure any grit gets washed off. Thinly slice into half-moons and transfer to a large mixing bowl. Add the squash, black-eyed peas, 3 tablespoons olive oil and 1 teaspoon salt. Arrange in an even layer around the pork and transfer the sheet pan to the oven.
Roast the pork for 25 minutes, or until lightly caramelized on top. Remove the pan from the oven and transfer the pork to a cutting board to rest under foil.
Redistribute the vegetables in an even layer and return to the oven to roast until caramelized, another 10 minutes.
Once it’s rested for 10 minutes, thinly slice the pork and transfer to a serving platter alongside the vegetables. Serve with the remaining sauce on the side.