Combine the dry spices with 1½ tablespoons of water in a small bowl, and mix to form a paste.
Put the oil in a medium saucepan over medium heat. When the oil shimmers, add the onions and sauté until the onions are browned at the edges (about 3-4 minutes).
Add the ginger and the paste of spices, stir to coat the onions, and cook for about 1 minute.
Add the diced tomato. Stir and cook, scraping the bottom of the pan, for about 4 minutes, until the tomato has softened. If the tomato sticks to the pan excessively, add a splash of water.
When the tomato has softened, add 2 cups of water and the salt. Stir and bring to a boil; then reduce the heat, cover, and simmer for 20 minutes.
Add the mushrooms. Bring the mixture back to a boil, reduce the heat again, and cook for 10 more minutes.
Add the peas, stir to combine, and cook for 5 more minutes over medium-low heat, until warmed through.
Salt to taste, and serve over basmati rice.