Sauté the vegetables
Melt butter in a skillet over medium heat.
Add diced onions, mushrooms, and jalapeño.
Sauté until the vegetables are softened and most of their moisture has cooked off, about 5–7 minutes.
Remove from heat and allow to cool slightly.
Prepare the meatloaf mixture
In a large mixing bowl, combine the milk, egg, breadcrumbs, Worcestershire sauce, and Italian seasoning.
Stir together to create a slurry. It should be moist and cohesive, but not runny or overly thick. Add a little milk or breadcrumbs if needed.
Add the sautéed onion, mushroom, and jalapeño mixture.
Season with salt and black pepper.
Add the ground beef and gently mix until everything is just combined. Avoid overmixing, which can make the meatloaf dense.
Make the glaze
In a small bowl, combine ketchup, mustard, and brown sugar.
Stir until the brown sugar is incorporated and the sauce is smooth.
Set aside.
Preheat oven to 375°F.
Lightly grease your preferred baking pan if needed. not needed with silicone.
Divide the meatloaf mixture between a silicone muffin pan, silicone mini loaf pan, or shape into one traditional loaf.
Spoon a generous layer of glaze over each portion or spread evenly over the top of the loaf.
Bake until the internal temperature reaches 160°F.