Pat steak strips dry and season liberally with cumin, chili powder, smoked paprika, salt, and pepper.
Heat 2 tbsp butter and 1 tbsp olive oil in a skillet over medium-high. Cook steak until browned and nearly cooked through (4-5 minutes), flip, add onions, and continue cooking until tender (6-8 minutes). Remove steak and onions; set aside.
In the same skillet, add remaining 1 tbsp butter. Cook garlic until fragrant (30 seconds). Add rice and stir to combine.
Pour in milk and mix while cheese cubes melt into a smooth sauce. Add cheddar cheese, stirring until fully incorporated and sauce thickens (5-7 minutes).
Return steak and onions to the skillet with rice and cheese. Cook together until rice absorbs moisture and cheese is evenly distributed (5-7 minutes). Stir in cilantro before serving.
Garnish with lime wedges, diced tomatoes, avocado slices, or pickled jalapeños.