Preheat oven to 400°F and line a sheet pan with parchment or foil. Cut broccoli into florets, cube the onion, and slice the tofu into thin pieces.
In a bowl, whisk together soy sauce, rice vinegar, miso, sesame oil, brown sugar, garlic, mustard, and sriracha. Add the tofu and gently toss to coat. Let it sit for a few minutes.
Spread the broccoli and onion on the sheet pan. Remove the tofu from the marinade and place it on the pan with the vegetables. Drizzle with oil and toss lightly, then spread everything into an even layer.
Bake for 15–20 minutes until the broccoli is tender and the tofu has lightly crisp edges.
While it cooks, whisk cornstarch with a small amount of water to make a slurry. Add the remaining water and reserved marinade, then cook over medium heat until thickened.
Pour the sauce over the cooked tofu and vegetables, toss to coat, and sprinkle with sesame seeds. Serve over rice while warm.
Notes
Press tofu well before cooking for better texture.
Spread ingredients out on the pan to help them roast instead of steam.
Use tamari for a gluten free option.
Best served fresh, but leftovers keep for a few days.