Heat the oil in a deep skillet over medium heat until it shimmers, then add the cumin seeds and let them crackle for about 30 seconds.
Add the chopped onion and cook, stirring occasionally, until translucent and fragrant—about 5 minutes. The onions should turn a soft, golden color.
Stir in the minced garlic and grated ginger, cooking for another minute until fragrant and slightly blistered at the edges.
Sprinkle in the turmeric and ground coriander, then toast the spices for 30 seconds until they release a warm, earthy aroma.
Pour in the canned diced tomatoes and stir well, scraping any bits from the bottom. Let the mixture simmer gently for about 10 minutes until the sauce thickens and deepens in color.
While the sauce simmers, peel and chop the potatoes into even 2 cm chunks. Add them to the sauce, stirring to coat, then cover and let simmer on low heat for about 15 minutes until the potatoes are tender and the sauce is fragrant.
Stir in the thawed and drained peas, cooking for another 3-5 minutes until they are heated through and vibrant green.
Season with salt to taste, then squeeze in fresh lemon juice and stir to brighten the flavors. Let the curry rest off heat for a few minutes to allow the flavors to meld beautifully.
Serve hot with rice or crusty bread, garnished with fresh herbs if desired, and enjoy the warm, comforting flavors of this classic, homey curry.