Bring a large pot of salted water to boil for the pasta. While waiting, prepare your mise en place: trim and cut the sirloin into 1-inch pieces, mince the garlic cloves, finely dice the onion, slice the shallots, slice the mushrooms, and measure out all your spices, broth, and other liquid ingredients. Starting the pasta now ensures it will finish cooking around the same time as your stroganoff, so everything comes together seamlessly.

Heat 3.5 tablespoons olive oil in a large skillet over medium-high heat. When shimmering, add half the beef pieces in a single layer and season with 1/2 teaspoon salt. Sear for 2-3 minutes without stirring to develop a golden crust, then flip and cook 1 minute more. Remove to a plate and repeat with the remaining beef. Don't worry about cooking the meat through—it will finish cooking later in the sauce, and searing it now builds crucial flavor through browning.

Add the butter to the same skillet and let it melt over medium heat. Add the sliced shallots and minced garlic, cooking for 1-2 minutes until fragrant. Then add the diced onion and cook for 2-3 minutes until softened. Sprinkle the thyme, remaining 1.5 teaspoons salt, black pepper, paprika, and cayenne over the vegetables, then add the flour and stir constantly for 1 minute to cook out the raw flour taste. This flour will help thicken your sauce naturally without any cornstarch.

Pour in the bone broth and use a wooden spoon to scrape up any browned bits stuck to the bottom of the skillet—this is pure flavor. Stir in the sliced mushrooms, spinach, worcestershire sauce, dijon mustard, and soy sauce. Simmer for 2-3 minutes until the mushrooms begin to soften and the spinach wilts down. The sauce should smell rich and savory at this point. I like to taste a small spoonful of the sauce here to check the seasoning before adding the meat back in.

Return the seared beef and any accumulated juices to the skillet, then cover and simmer for 2-3 minutes. Uncover and cook uncovered for another 2-3 minutes, stirring occasionally, until the beef is cooked through and the sauce has thickened slightly. Remove the skillet from heat and let it cool for just 30 seconds—this is important so the yogurt doesn't curdle when you add it.

Stir the plain Greek yogurt into the stroganoff until fully incorporated, then taste and adjust seasonings as needed. Drain the cooked pasta and add it directly to the skillet, tossing gently to coat everything in the creamy sauce. Serve immediately in bowls. The Greek yogurt adds a tangy richness without the heaviness of traditional sour cream, making this stroganoff feel lighter while still being deeply satisfying.