Press the firm tofu between paper towels for 15 minutes to remove excess moisture, then cut into 1-inch cubes.
Heat the toasted sesame oil in a large skillet or wok over medium-high heat.
Add the tofu cubes to the skillet and sear until golden brown and crisp on all sides, then remove the tofu and set aside.
In the same pan, add the sliced yellow onion, cremini mushrooms, and red bell pepper, sautéing for 5-6 minutes until the vegetables are tender-crisp.
Stir in the minced garlic and fresh ginger, cooking for 1 minute until the mixture becomes highly fragrant.
Return the seared tofu to the pan and pour in the tamari, sea salt, and black pepper.
Add the baby spinach to the skillet and toss gently for 1-2 minutes until the leaves are just wilted.
Serve the stir-fry immediately while hot and steaming.