Heat a large skillet over medium heat. Once hot, add the olive oil and saute the garlic, onions and peppers until soft and lightly browned, about 8 minutes. Add the chili powder, and some salt and pepper and stir until fragrant, about 1 minute. Add the diced tomatoes with their juices and 1/2 cup water and simmer on low for 10 minutes. Season the mixture with a big pinch of salt and healthy grind of black pepper. Let cool.
Add the cooled tomato mixture and the lime juice into a food processor or blender and pulse a few times, keeping some of the texture. Remove and reserve 1/2 cup of the salsa for serving. Add the remaining salsa to a 13-by-9-inch casserole dish. Add the skirt steak and flip to make sure it's well coated. Cover with plastic wrap and place in the fridge for at least 1 hour, or up to 8.
Preheat a grill to medium-high heat. Take the steak out of the refrigerator and bring to room temperature while the grill is heating.
Once the grill is hot and ready, remove the steak from the salsa marinade and shake off any excess. Grill for 4 minutes on each side for medium. Rest for 10 minutes. Wrap the corn tortillas in aluminum foil and heat on the grill until warm and soft.
To serve, slice the steak into 4-inch sections, and then slice into 1/4-inch-thick slices across the grain. Serve the sliced steak in the warm corn tortillas, topped with the reserved salsa and Guacamole.
For kid's taco toppings: Cut the steak into smaller bite-size pieces. In place of the salsa, top the tacos with chopped Roma tomatoes or purchase a mild store bought red salsa. Top kid's tacos with shredded Cheddar, sour cream and shredded iceberg lettuce.
Pit and peel the avocados. Place the avocados in a bowl and mash with the lime juice, onions, sour cream and a big pinch of salt and pepper. Fold in the tomatoes and serve.