Seasoning with salt at the end of the cooking process may seem unconventional, but it actually works better in stir-fries. Salt encourages vegetables to break down and release water, and that is not the goal in these dishes.
Heat the canola oil with the fenugreek leaves and chiles (if using), cumin seeds, coriander, ground cumin, and turmeric in a large wok, Dutch oven, or 12-inch skillet over medium-high heat. Cook, stirring occasionally, until the cumin is browned and the chiles darken, 2 to 3 minutes. Add the mushrooms, onions, and tomatoes, mix them into the spices, and cook, stirring occasionally, until the liquid has evaporated and the vegetables look evenly browned, softened, and dry, 15 to 20 minutes; there will be a lot of liquid in the beginning— just keep cooking, stirring more frequently as the vegetables become drier.
Add the garam masala, cayenne (if using), and salt, and cook 1 minute longer. Stir in the cilantro and lemon juice, taste for seasoning, and serve.