Press the tofu for at least 20 minutes to remove excess moisture, then cut into evenly sized cubes.
Heat the vegetable oil in a large wok or skillet over medium-high heat until shimmering and slightly smoky.
Gently place the tofu cubes into the hot oil, laying them away from you to avoid splatter, and let them cook undisturbed for 3–4 minutes until the edges turn golden brown and crispy.
Use spatula or tongs to carefully flip the tofu pieces, then cook for another 2–3 minutes until all sides are evenly crisped and golden.
Remove the tofu from the pan and set aside on a plate lined with paper towels to absorb excess oil.
Add the minced garlic and sliced bell peppers to the same hot pan, cooking for about 1 minute until fragrant and peppers are slightly softened but still vibrant and crisp.
Return the crispy tofu to the pan, pouring in the soy sauce and sprinkling chili flakes if using. Toss everything gently to coat evenly and cook for another 2 minutes until the sauce thickens slightly and becomes glossy.
Finish by drizzling sesame oil over the stir-fry, tossing gently to distribute the aroma and flavor evenly.
Transfer the stir-fry to a serving dish and let it rest for 2 minutes to settle the flavors.
Serve immediately while hot, with the crispy tofu edges crackling and the peppers still vibrant and crisp.