Season the steaks with salt and pepper on all sides; let the steaks sit at room temperature for 10 minutes.
In a large, heavy-bottomed skillet, heat 2 tablespoons avocado oil over medium-high heat. Sear the steaks for 2 minutes. Flip and sear for 2 more minutes. Don't crowd the skillet; cook the steaks in batches if your skillet isn't big enough.
Remove the steaks from the pan and set aside on a plate; keep them covered.
Add the remainder of the avocado oil to the pan. Add the onions and bell peppers to the pan. Season with salt and sauté until softened, about 5 minutes, frequently stirring.
Stir in the garlic and cook for 15 seconds or until fragrant.
Add the wine to the pan. Stir to scrape up any browned bits off the bottom and sides of the pan and into the sauce. Let the wine cook until it evaporates, about 30 seconds.
Add the crushed tomatoes, olives, and capers. Bring the sauce to a boil; then reduce the heat to a steady simmer.
Add the steaks to the pan and cook for 3 minutes. Flip the steaks and cook for an additional 2 minutes.
Remove from heat and transfer the steaks and sauce to a serving platter.
Serve hot with crusty bread, pasta, and a light salad.
Notes
Sirloin Steaks: Each steak should be boneless and 1-inch thick, at the most, and weigh about 4 to 5 ounces each. If the steaks are thicker, use a meat mallet to pound them down.
Refrain from crowding the pan when searing the steaks because they will not sear properly and will likely be tough and chewy.
I prefer using avocado oil for searing and sauteeing, but you can use vegetable oil, canola oil, or any other cooking oil with a high smoke point.
I suggest using your favorite drinking white wine. Pinot Grigio or Chardonnay are my favorite choices. If you don't want to use wine, you can use chicken broth in its place.
When finishing the steak in the tomato sauce, cook over medium-low to low heat. You want the sauce to be just simmering.