For the sauce: Whisk together all of the sauce ingredients (soy sauce through sriracha). Set aside.

For the stir fry: Bring a large pot of salted water to a boil. Add udon noodles and cook according to package directions. Drain noodles and rinse under cold water. Take care to not overcook the noodles, as they can become mushy!

Heat a large skillet over medium-high heat. Add ground beef and cook, stirring occasionally, until the meat is fully browned, about 5-8 minutes. Remove beef to a bowl and discard any grease.

Add sesame oil and butter to the skillet used to cook the beef (no need to wash it). Set the skillet over medium-high heat. When the butter has melted, add the sliced shiitake mushrooms and saute them for 2-3 minutes per side, or until they are golden brown. Remove mushrooms to the bowl with the beef.

Add the prepared sauce to the skillet and bring it to a boil over medium-high heat. Reduce heat to medium and add the cooked udon noodles. Toss to combine (a tongs works great for this).

Add the beef, mushrooms, snap peas, and green onions to the noodles. Toss to combine and cook for 1-2 minutes, or just until the snap peas are warmed through and the sauce evenly coats the vegetables.

Serve immediately, garnished with sesame seeds and additional green onion. Store any leftovers in an airtight container in the fridge for up to three days.