For the beef: In a skillet over medium-high heat, add the olive oil and onions. Cook until starting to soften, then add the ground beef. Cook the meat until it's totally browned, then drain the fat. Add the chili powder, cumin, paprika, salt, black pepper and crushed red pepper and stir to combine. Add 1/2 cup hot water and stir. Reduce the heat to low and simmer for 15 minutes.
For the fixins: Meanwhile, wrap the flour tortillas in foil and warm in the oven for 20 to 25 minutes. Right before serving, crisp the taco shells in the oven according to package instructions.
Serve with the grated cheese, diced tomatoes and shredded lettuce.