Cook the Vegetables: Melt 2 tablespoons of butter in a medium skillet over medium heat. Sauté 2 cups of finely chopped cremini mushrooms for about 5 minutes until they release moisture. Add 1 cup of finely chopped onion and 1 large finely chopped shallot, cooking until softened, approximately 5 to 10 minutes. Next, stir in 1/2 cup of finely chopped red bell pepper, 3 minced garlic cloves, and 1/4 teaspoon of optional Cajun seasoning. Cook together for an additional 5 minutes before allowing the mixture to cool for 10 minutes.

Mix the Meatloaf: In a large mixing bowl, combine the cooled vegetable mixture with 1 pound of ground beef. Add in 1/2 cup of finely crushed saltine crackers, 1/4 cup of milk, 1 large egg, 1 egg yolk, 1 teaspoon of whole-grain mustard, 1 teaspoon of ketchup, and 1 teaspoon of Worcestershire sauce. Mix until well blended but avoid overmixing.

Prepare the Glaze: In a separate small bowl, whisk together 1/2 cup of ketchup, 1 teaspoon of maple syrup, 1 teaspoon of whole-grain mustard, and 1 teaspoon of Worcestershire sauce until smooth.

Form the Loaf: Shape the meat mixture into a loaf approximately 9x5 inches and place it on a prepared baking sheet. Spread the glaze over the top of the meatloaf.

Bake: Preheat your oven to 375°F (190°C). Bake for 45 to 55 minutes or until the internal temperature reaches 160°F. Let it rest for about 10 minutes before slicing.