Bring a large pot of salted water to a boil and cook the gluten-free noodles according to the package directions until just al dente. Drain well and set aside.
While the noodles cook, pat the beef sirloin strips dry with paper towels and season them lightly with a pinch of the salt and black pepper.
Heat 1 tablespoon of olive oil in a large skillet over medium-high heat. Add half of the beef strips in a single layer and sear for 1 to 2 minutes per side until browned but not fully cooked through. Transfer the beef to a plate and repeat with the remaining 1 tablespoon of olive oil and beef strips.
Reduce the heat to medium and add the unsalted butter to the same skillet. Once melted, add the sliced yellow onion and cook, stirring often, for 4 to 5 minutes until softened and lightly golden, scraping up any browned bits from the bottom of the pan.
Stir in the minced garlic and cook for 30 seconds to 1 minute until fragrant, being careful not to let it burn.
Pour in the gluten-free beef broth, gluten-free tamari or soy sauce alternative, and Worcestershire sauce. Add the dried thyme, dried parsley, remaining salt, and black pepper. Stir well to combine and bring the mixture to a gentle simmer.
In a small bowl, whisk together the cornstarch and cold water until completely smooth with no lumps, creating a slurry.
Slowly pour the cornstarch slurry into the simmering broth mixture while stirring constantly. Continue to simmer for 2 to 3 minutes, stirring, until the sauce thickens into a glossy gravy that coats the back of a spoon.
Return the seared beef strips and any accumulated juices to the skillet, stirring to coat them in the gravy. Let the beef simmer gently in the sauce for 3 to 4 minutes until cooked through and tender.
Add the cooked gluten-free noodles to the skillet, tossing gently with tongs or a large spoon until the noodles are evenly coated in the beef gravy and everything is well combined.
Sprinkle the chopped fresh parsley over the beef and noodles, give everything a final gentle toss, and taste to adjust seasoning with additional salt or pepper if desired.
Remove the skillet from the heat and let the beef and noodles rest for 2 to 3 minutes so the flavors meld, then serve warm straight from the pan.