Bring a pot of salted water to a boil and cook the whole wheat penne according to package directions until al dente.
While pasta cooks, season the cubed sirloin steak with sea salt, black pepper, and half of the Cajun seasoning.
Heat the olive oil in a large skillet over medium-high heat and sear the steak pieces for 2-3 minutes until browned, then remove from the pan.
In the same skillet, add the sliced bell peppers and diced onion, sautéing for 4-5 minutes until the vegetables are tender-crisp.
Stir in the minced garlic, tomato paste, and the remaining Cajun seasoning, cooking for 1 minute until fragrant.
Pour in the beef broth to deglaze the pan, scraping up any browned bits, and simmer for 2 minutes.
Remove the skillet from heat and let it cool slightly before stirring in the Greek yogurt to create a smooth, creamy sauce.
Toss the cooked pasta and seared steak back into the skillet, stirring until everything is well-coated in the spicy sauce.