Heat a medium heavy-bottomed pan over medium heat and add a tablespoon of oil. Once shimmering, sprinkle in the mustard seeds; wait until they crackle and pop, about 30 seconds.
Add the chopped onion to the pan and sauté for 5-7 minutes, stirring often, until it turns golden brown and fragrant.
Stir in the minced garlic and grated ginger, cooking for another 1-2 minutes until aromatic and well combined.
Add the garam masala and turmeric, toast for 30 seconds while stirring, until spices release a warm, fragrant aroma.
Pour in the coconut milk and water, stirring to combine and bring to a gentle simmer.
Stir in the frozen green peas directly from the freezer, ensuring they are evenly distributed in the sauce.
Simmer uncovered for 5-8 minutes, stirring occasionally, until the peas are tender and the sauce has thickened slightly, turning a glossy, vibrant green.
Taste the curry and season with salt as needed. Remove from heat and let it rest for a couple of minutes to meld the flavors.
Finish by sprinkling chopped cilantro over the top and squeezing a little lemon or lime juice for brightness.
Serve the hot green pea curry over steamed rice or with flatbread, enjoying its bright, velvety texture and sweet, earthy flavors.