Combine ground flaxseed with 5 Tbsp. water in a small bowl. Let sit until the mixture becomes gelatinous.Preheat oven to 375ºF and thoroughly grease 10 cups of a 12-cup muffin tin.
Heat oil in a large skillet over medium-high heat. Once hot, add onions and mushrooms; cook until softened, stirring occasionally, about 8 to 10 minutes. Add tomato paste, Italian seasoning, smoked paprika, and garlic powder; stir well to combine. Cook 2 more minutes, allowing the tomato paste to deepen in color. Add Worcestershire sauce, brown sugar, salt, and pepper; stir well to combine. Turn mixture out into a large bowl.
Add walnuts to the bowl of a food processor and blend until they resemble a very coarse sand. Add lentils and pulse about 6 times, just to break down but NOT puree them. (It’s important to maintain some texture to ensure the meatloaf isn’t mushy. Ideally, you’ll still have some whole lentils at the end.) Empty lentil mixture into bowl with mushroom mixture.
Add breadcrumbs and flax eggs, and use a rubber spatula (or your hands) to thoroughly mix together. (The mixture will be thick, however this ensures the mini loaves stay together when baked.)
Divide the meatloaf mixture among the 10 greased muffin cups, gently pressing to fill. (I like to smooth the tops and shape them slightly like a dome.) Bake for 25 minutes.
Meanwhile, prepare BBQ glaze by combining all ingredients in a medium bow; whisk well to combine. Remove meatloaves from the oven and spoon the BBQ glaze evenly over each one. Place pan back in the oven and bake for 10 more minutes. Let sit for 10 minutes before using an offset spatula (or butter knife) to remove meatloaves from the pan.