Mia's is right down the street from my parent's home. I must have eaten there a thousand times growing up and have become close friends with the "Mia's family". I can say, without a question, that Mia's brisket tacos are the ultimate culmination of Texas and Mexican style cuisines. The Texas slow-cooked brisket wrapped inside flour tortillas with slices of avocado and cotija cheese with the gravy spilling out of the ends into the obligatory and heaping sides of Tex-Mex rice and refried beans is beyond compare. As for the gravy, it's not a white gravy--it is hardly more than the cooking juices of the brisket. If you cook the brisket in a slow-cooker with the same idea as the "Chili Gravy" recipe using chile powder, cumin, oregano, garlic, salt, and pepper as a rub, some broth or water, and add some stewed tomatoes and sliced onions, but no flour or oil, you will have something very close to Mia's brisket tacos.
Good luck from Texas!