Heat olive oil in a large skillet over medium-high heat until shimmering, about two minutes. Working in batches, sear beef cubes for 90 seconds per side until deeply browned; transfer to your slow cooker.

Add diced onion and sliced mushrooms to the same skillet, scraping up the flavorful brown bits from the bottom. Sauté for four minutes until softened, then add minced garlic and cook 60 more seconds until fragrant.

Stir tomato paste into the skillet, coating all vegetables evenly for 90 seconds to deepen the flavor. Add Worcestershire sauce, beef broth, thyme, oregano, salt, and pepper to the skillet, stirring until combined.

Pour the vegetable mixture over the seared beef in your slow cooker and stir gently to combine. Cook on low for 8 hours—the beautiful slow cooker beef stroganoff recipe requires patience; do not rush this step or beef stays tough.

In a small bowl, whisk cornstarch with three tablespoons cold water until completely smooth with no lumps. Thirty minutes before serving, stir the cornstarch slurry into the slow cooker, whisking it through the liquid completely.

Remove from heat and let rest for five minutes, then stir in sour cream gently to avoid breaking the silky texture. Taste and adjust seasoning with salt and pepper as needed.

Cook egg noodles according to package directions in salted boiling water, drain, then toss with butter. Divide noodles among serving bowls and ladle the stroganoff over top.