Heat the oil in a large skillet over medium heat until shimmering. Add the cumin seeds and fry until fragrant, about 30 seconds, listening for a gentle crackle.
Add the finely chopped onions and cook, stirring frequently, until they turn translucent and start to golden, about 8-10 minutes. You'll smell a sweet aroma as they soften.
Stir in the minced garlic and grated ginger, cooking for another minute until fragrant. Add the turmeric, garam masala, and smoked paprika (if using), and toast the spices until they release a deep aroma, about 30 seconds to 1 minute.
Add the diced tomatoes and cook, stirring occasionally, until they break down into a thick sauce and the mixture smells bright and tangy, about 5-7 minutes. If it seems watery, turn up the heat slightly to help it reduce.
Introduce the sliced mushrooms into the skillet, stirring well to coat them with the spice mixture. Cook for 10-12 minutes, stirring occasionally, until the mushrooms release moisture and then absorb the flavors, turning darker and more tender.
Check the consistency: if the sauce is too thin, simmer a few more minutes until thickened. If it looks too dry, add a splash of water or sprinkle in a little more oil to keep it luscious.
Stir in the heavy cream or coconut milk if using, cooking for another 2 minutes until heated through and velvety. Taste and season with salt and a squeeze of lemon juice for brightness.
Remove from heat and let the mushroom masala rest for a couple of minutes to deepen the flavors. Garnish with chopped cilantro if desired, then serve hot with rice or naan.
