Arrange a rack in the middle of the oven and heat the oven to 375ºF.
Peel 1 medium white or yellow onion and grate on the large holes of a box grater into a large bowl (about 3/4 cup). Strip the leaves from 6 large fresh thyme sprigs and finely chop to get 2 teaspoons. Add to the bowl.
Add 1 cup quick-cooking oats, 1/3 cup whole milk, 2 large eggs, 1 tablespoon kosher salt, 1 teaspoon black pepper, and 1 teaspoon dry mustard, and stir to combine. Add 2 pounds lean ground beef and use your hands to gently mix everything until evenly combined. Transfer the mixture to a 9×5-inch loaf pan and smooth the top with a spatula.
Bake for 45 minutes. Meanwhile, make the glaze: Place 1/3 cup ketchup, 2 tablespoons Dijon mustard, 1 tablespoon packed light brown sugar, 2 teaspoons Worcestershire sauce, and 1 teaspoon hot sauce, if using, in a small bowl and stir to combine.
Remove the meatloaf from the oven. Pour the glaze over the top and use a spatula to smooth it out. Return to the oven and bake until the glaze is slightly caramelized and the internal temperature of the meatloaf registers at least 165ºF, 10 to 25 minutes more.
Let the meatloaf cool in the pan for 15 minutes. Pour off the excess grease into a small bowl if desired. Use a sturdy spatula (like a fish spatula) to transfer the meatloaf from the pan to a clean cutting board. Cut into slices and serve.