Heat the oil in a large skillet over wok over medium-high heat.
Add the bratwurst pieces and cook, stirring often, until lightly browned and getting crispy around the edges, 5 or so minutes. Remove with a slotted spoon to a plate or small bowl.
Add the potato chunks and cook for 5 minutes or until just starting to soften. Remove to the plate with the bratwurst.
Add the squash and cook until just tender, 2-3 minutes. Remove to the plate with the bratwurst.
Add the bell peppers and onions and cook until tender, stirring, 3-4 minutes.
Add the bratwurst, potatoes and squash back to the skillet.
Stir in the garlic and cook 3 minutes.
In a small bowl, whisk together the mustard, beer and Worcestershire sauce. Add to the meat mixture.
Reduce heat to medium-low. Simmer slowly, stirring occasionally, until the sauce reduces slightly. Taste and add salt and pepper to taste. You may also want to add more mustard.
Serve garnished with the green onions.
Notes
You can use any combination of vegetables you want. Always start cooking the thicker, harder vegetables (such as carrots) first since they take longer to cook.