Preheat the oven to 400°F (200°C). Arrange the corn tortillas on a large baking sheet and lightly spray both sides with cooking spray; season lightly with salt.
Bake the tortillas until the underside is lightly browned and crisp, about 3–5 minutes, then flip and bake the other side until lightly browned and crisp, about 3–5 more minutes. Set aside to cool.
Heat the olive oil in a large heavy-bottom skillet over medium-high heat. Add the diced onion and cook until softened, about 5 minutes.
Add the minced garlic and cook 1–2 minutes until fragrant, then stir in the diced red bell pepper and cook until tender, about 3–4 minutes.
Add the cubed, drained tofu to the skillet. Sprinkle with chili powder, ground cumin, salt and pepper to taste, then squeeze the juice of 1 lime over the tofu. Cook, turning occasionally, until the tofu is browned on multiple sides, about 5 minutes.
Stir in the 1/4 cup chopped fresh cilantro and remove the skillet from the heat.
Top each crisp tostada shell with a portion of the tofu mixture. Garnish with halved grape tomatoes, diced avocado, additional chopped cilantro, and a squeeze of lime juice if desired.