Combine chili powder, sea salt, and cumin in a bowl; rub over steak. Place steak on a plate and cover with plastic wrap; refrigerate for 1 hour and up to overnight. Remove from refrigerator and let sit at room temperature for 30 minutes.
Heat a grill pan over high heat. Rub steak and sliced yellow onion with vegetable oil; cook on the hot grill until steak is cooked through, 3 to 4 minutes per side. An instant-read thermometer inserted into the center should read 140 degrees F (60 degrees C). Let steak and onion rest for 20 minutes. Thinly slice.
Preheat oven to 425 degrees F (220 degrees C). Spread onion rings in 1 layer onto a baking sheet.
Bake in the preheated oven until browned and crisp, 14 to 16 minutes.
Cook and stir cheese food and diced tomatoes and green chiles in a saucepan over medium-low heat until cheese sauce is smooth, 5 to 10 minutes.
Spread baked onion rings on a serving platter and top with steak and onions. Pour 3/4 of the cheese sauce over the top, reserving the remaining sauce to serve alongside the nachos. Top nachos with tomato, cilantro, jalapeno peppers, and green onions.