Bring a large pot of salted water to a boil for the tortellini. Cook the cheese tortellini according to package directions until al dente. Drain immediately, reserving at least 1 cup of the starchy pasta water. While the tortellini cooks, cube the sirloin steak and season aggressively with salt and pepper.

Heat the olive oil in a large, deep skillet or Dutch oven over medium-high heat. Once the oil is shimmering, add the seasoned steak cubes. Cook in a single layer, without overcrowding, for 2-3 minutes per side until beautifully caramelized and browned. Remove the steak from the pan and set it aside, leaving all the rich, browned bits (the fond) in the skillet.

Reduce the heat to medium-low. Add 2 tablespoons of butter to the skillet. Once melted, add the minced garlic and the optional chopped yellow onion. Sauté, stirring constantly, for about 60-90 seconds until extremely fragrant. Be careful not to burn the garlic.

Pour in the chicken broth, scraping up all the savory browned bits from the bottom of the pan with a wooden spoon. Bring this mixture momentarily to a simmer, allowing it to reduce slightly.

Reduce the heat to low. Whisk in the heavy cream and the remaining 2 tablespoons of butter. Turn off the heat completely and slowly stir in the grated parmesan cheese until smoothly incorporated and the sauce is uniform and velvety. Taste and adjust seasonings.

Return the seared steak cubes to the sauce. Add the drained cheese tortellini. If the sauce looks too thick, add a splash or two of the reserved pasta water until the sauce coats everything. Garnish with fresh parsley or basil and extra parmesan before serving.