You can substitute Chinese broccoli with broccolini or dark, leafy greens such as kale, turnip, or mustard greens in this vegetarian-friendly dish.
Press tofu: Slice tofu into 1-inch slabs, and place in a single layer on a baking sheet lined with cheesecloth or paper towels. Cover with another layer of cheesecloth or paper towels, and place another baking sheet or plate on top. Weight evenly with canned goods or other heavy items. Let stand about 30 minutes. Drain off liquid, and pat tofu dry with paper towels.
In a large shallow dish, combine mustard, sesame seeds, garlic, and soy sauce. Place tofu in dish, and turn once to coat evenly with marinade. Let marinate at least 20 minutes at room temperature while preparing ingredients for stir-fry.
Heat a 12-inch nonstick saute pan or a wok over medium-high heat, and add 1 1/2 teaspoons canola oil to pan, swirling pan to coat. Add tofu; cook until lightly browned on each side, about 1 1/2 minutes per side. Transfer tofu to a platter; cover loosely with aluminum foil to keep warm.
In same pan or wok, heat remaining 2 teaspoons canola oil. Add ginger and garlic; cook, stirring constantly, until aromatic, about 30 seconds. Add mushrooms, red pepper, Chinese broccoli, and scallions; cook, stirring constantly, until vegetables are crisp-tender and bright, about 7 minutes. Add soy sauce and sesame oil, and stir to combine. Season with pepper, and serve immediately spooned over the tofu.
Tamari is a thicker, milder version of soy sauce. Before marinating tofu, press it as described in step one to remove excess moisture.