Cut and dry the steak. Slice the sirloin into small, even 1-inch cubes. Pat the pieces very dry with paper towels so they sear instead of steam.
Season well. Add steak bites to a bowl. Sprinkle with 1 teaspoon salt, black pepper, smoked paprika, and onion powder. Toss until every piece is evenly coated.
Heat the skillet. Place a large skillet over medium-high heat for 2 minutes. Add the oil and let it shimmer before adding the steak.
Sear in batches. Add half the steak bites in a single layer. Cook for 2 minutes without moving them, then turn and cook for another 1 to 2 minutes until browned on multiple sides. Transfer to a plate. Repeat with the remaining steak.
Lower the heat. Reduce heat to medium-low. Add butter to the same skillet and let it melt, scraping up the browned bits with a spoon for extra flavor.
Cook the garlic. Add minced garlic and thyme. Cook for 30 seconds until fragrant, being careful not to burn.
Finish the sauce. Stir in remaining 1/4 teaspoon salt, chopped parsley, and lemon juice. Simmer the sauce for 1-2 minutes until glossy.
Return steak to skillet. Add the seared steak bites back to the skillet, tossing them in the garlic butter sauce until fully coated.