Mushroom Stir-Fry with Onions & Tomatoes

1/4 cup dried fenugreek leaves or 1/2 cup finely chopped fresh fenugreek

2 small whole dried red chiles

1 Tbs. cumin seeds

2 tsp. ground coriander

1 tsp. ground cumin

1/2 tsp. ground turmeric

2 lb. cremini (baby bella) mushrooms, cleaned, stems trimmed, and sliced

1/4 inch thick (8 cups)

2 medium red onions, peeled, halved lengthwise and thinly sliced crosswise into half-moons (3 cups)

2 medium tomatoes, finely chopped (about 2-1/2 cups)

1 tsp. store-bought or homemade garam masala

1/4 tsp. cayenne

2 tsp. kosher salt; more to taste

1/4 cup fresh cilantro, chopped

1 to 2 Tbs. fresh lemon juice