Heat oil in a wide skillet over medium heat until shimmering. Add cumin seeds and toast until fragrant, about 30 seconds.
Add the finely chopped onion to the pan. Cook, stirring frequently, until translucent and just starting to brown—about 5 minutes. The onions should smell sweet and become slightly caramelized.
Stir in the minced garlic and cook for another 30 seconds until fragrant, making the kitchen fill with a pungent aroma.
Sprinkle in turmeric and chili powder, stirring constantly for about 1 minute until the spices release their aroma and darken slightly.
Add the sliced mushrooms to the pan. Stir well and cook, stirring frequently, until they release their moisture and start to shrink—around 8 minutes. The mushrooms should look glossy and darkened at the edges.
Pour in the chopped tomato and stir to combine. Cook until the tomato softens and the mixture thickens, about 4 minutes. The aroma should be bright with a hint of tang.
Season with salt to taste. Reduce heat to low, cover the pan, and let everything simmer gently for 10 minutes, stirring occasionally. The mushrooms will be tender and flavors melded.
Remove from heat and squeeze a little lemon over the bhaji for a fresh brightness, if desired. Let rest for 2 minutes to allow flavors to deepen.
Serve the mushroom bhaji warm, accompanied by bread or rice, and enjoy its earthy, spicy richness.