Preheat a grill or grill pan to medium high. Mix the yogurt, garlic and cilantro in a small bowl; set aside.
Toss the onion rings with the olive oil and a pinch of salt. Sprinkle the steak with 1/2 teaspoon salt. Lightly brush the grill with olive oil and grill the onion and steak, turning once, until the onion is charred and the steak is cooked to medium rare, about 5 minutes per side. Transfer the steak to a cutting board and let rest 5 minutes.
Make the slaw: Put the carrots and bell pepper in a medium bowl and toss with the jalapeno, lime juice and 1/4 teaspoon salt.
Wrap the tortillas in a damp paper towel, then in plastic wrap; microwave until warm, 1 minute. Thinly slice the steak against the grain. Divide the steak and onion among the tortillas; top with the slaw, yogurt sauce and more cilantro. Serve with lime wedges.