Bring a large pot of salted water to a boil. Add the penne pasta and cook according to the package instructions until al dente, usually about 10-12 minutes. Drain the pasta and set aside, reserving a small cup of pasta water for later.
Heat 2 tablespoons of olive oil in your large skillet over medium-high heat. Season the steak strips generously with salt and pepper. Once the oil is shimmering, add the steak strips in a single layer. Sear for about 2-3 minutes per side until browned but still juicy inside. Remove the steak from the skillet and set aside.
Reduce the heat to medium. Add the remaining tablespoon of olive oil to the skillet. Toss in the minced garlic and sauté for about 30 seconds until fragrant. Add the chopped spinach and cook until wilted, about 2 minutes, stirring frequently.
Pour the heavy cream into the skillet with the garlic and spinach. Stir well to combine. Bring the cream to a gentle simmer and cook for 3-4 minutes to slightly thicken. Then stir in the grated Parmesan cheese until melted and smooth. Season with salt, pepper, and red pepper flakes if using.
Add the cooked penne pasta and seared steak strips back into the skillet with the sauce. Toss everything gently to coat the pasta and steak evenly. If the sauce feels too thick, add a splash of the reserved pasta water until you reach your desired consistency.
Once everything is heated through and coated in that creamy sauce, remove the skillet from heat. Serve immediately, garnished with extra Parmesan or fresh cracked pepper if desired. Enjoy the rich flavors and comforting texture of this all-in-one skillet meal.