Peel and grate the ginger and garlic into a prep bowl. Use a masher, fork, or back end of a spoon to combine into a paste.
Heat the oil in a large skillet on medium heat. Add in the onions, cook until onions start to take on color, stirring occasionally, about 10 minutes. Add garlic/ginger paste, chili peppers, and mustard. Cook until onions start to take on a reddish brown color, stirring occasionally but a little more frequently now. (may have to deglaze pan). About 10 more minutes here.
Add the coriander, cumin, and garam masala, stirring and until fragrant. Add in the lamb, mint, and turmeric and cook until just browned.
Stir in the fire roasted tomatoes (with liquid). Allow liquid to reduce. Stir in half the cilantro.
Fill the mushrooms, with mixture, Bake for 30 minutes at 350°, covered. Remove foil or covering, sprinkle feta cheese on top. Cook uncovered for another 7 - 10 minutes or until cheese starts to soften/melt. Remove, top with remaining cilantro for garnish and serve.