Put the oil in a large skillet over medium-high heat. Add the onion and cook, stirring occasionally, until it’s translucent, 1 or 2 minutes.
Add the broccoli and 1/4 cup water and sprinkle with salt and pepper. Cook, stirring occasionally, until the broccoli is brightly colored but still crisp and the pan is dry, 3 to 5 minutes.
Crumble the tofu into the pan and stir, using a spatula to scrape the bottom of the pan and combine the tofu and vegetables; adjust the heat as necessary to avoid burning. When the mixture starts to stick to the pan, add the lemon juice, taste and adjust the seasoning, and serve hot or warm.