Layer 2 folded dish towels on a cutting board and place tofu on top. Place 2 more folded dish towels and a heavy book on top. Press tofu for 5 to 10 minutes or longer. Cut into bite-sized cubes.
Heat a large nonstick pan over medium heat. Add tofu and dry-fry until golden on 1 side, about 5 minutes. Flip and cook until golden on the other side, about 5 minutes more.
While tofu is frying, whisk soy sauce, agave nectar, Dijon mustard, ginger, sesame oil, and cayenne pepper together in a bowl. Stir in the fried tofu, coating thoroughly in the marinade. Cover the bowl and refrigerate about 30 minutes or longer.
Combine water and quinoa in a small saucepan and bring to a boil. Reduce heat to medium and simmer until quinoa is fluffy and water has been absorbed, 13 to 15 minutes. Monitor the quinoa while it cooks; add extra water if necessary.
While quinoa is cooking, place sweet potatoes in a small saucepan and cover with 1 or 2 inches of water. Bring to a boil and cook for 3 minutes. Drain.
Combine cooked quinoa, tofu and marinade, cooked sweet potatoes, onion, and peanut oil in a large frying pan over medium-high heat. Cook, stirring constantly, until onion is soft, 3 to 4 minutes.