Preheat oven to 400 degrees F. Line a large baking sheet with foil and grease lightly with butter.
To a large mixing bowl add the sirloin, onion, garlic, tomatoes, tortilla crumbs, eggs, Worcestershire, cheese, and spices including about 1/2 teaspoon coarse salt and about 1/8 teaspoon black pepper. Use a fork to combine.
Divide meat into 4 equal portions right in the bowl. Lay a flour tortilla out and add a portion of the meat a few inches from the edge - form into a "loaf" shape. Fold the end over the meat and then the sides, roll up and place on baking sheet seam side down. Repeat until all the burritos are made.
Melt 2 tablespoons of butter and use a pastry brush to coat the burritos all over. Bake 25-30 minutes to an internal temperature of 165 degrees F. Check the burritos halfway through and if they are starting to brown too much cover them with foil.
To make the topping heat a medium-size skillet over medium-low heat and melt butter and olive oil together. Add onion and cook until soft and browned, stirring often. Toss with the avocado, season with salt and pepper.
To serve slice meatloaf with a serrated knife into 1" thick rounds. Garnish with onion mixture, fresh herbs and sour cream.