Heat olive oil in a large saucepan over medium-high heat. Cook and stir onion in the hot oil until soft and beginning to brown, about 12 minutes. If browned bits of onion are stuck to the bottom of the pan, stir water into onion and stir to loosen the browned bits.
Mix ground beef, garlic, ginger, serrano chile, and cilantro into pan; cook and stir until beef is browned and crumbly, 10 to 15 minutes. Reduce heat to medium-low. Stir coriander, salt, cumin, cayenne pepper, and turmeric into the beef; cook and stir until flavors blend, about 5 minutes. Add tomatoes and potatoes, cover pot, and simmer until potatoes are tender, about 15 minutes.
Mix green peas into dish and cook until sauce has slightly thickened and flavors have blended, 10 to 15 minutes. Sprinkle garam masala over the dish, cover, and let stand for 5 minutes before serving.
