1. Warm oil in a large skillet over medium-high heat until shimmering. Add mushrooms, snap peas and scallions and cook, stirring constantly, until snap peas are bright green and mushrooms have released their liquid, 3 to 4 minutes. Add garlic and cook, stirring, until golden, about 30 seconds.
2. Add soy sauce and cook for 3 to 4 minutes more, stirring, until snap peas are crisp-tender. Sprinkle with red pepper, if desired. Serve.