Tomato chutney (store-bought), for serving
Preheat the oven to 425 degrees F. In a large bowl, toss potatoes and onions with 3 tablespoons of the olive oil, along with the mustard seed, turmeric, cumin seed, black pepper, and salt. Spread in a single layer on a rimmed baking sheet. Roast potatoes, until tender, about 25 minutes.
Toss shrimp with 1 tablespoon of olive oil and the ginger and garlic. Season with salt and pepper. Scatter the shrimp and peas over the potatoes and continue to roast until shrimp are opaque and cooked through, 7 to 10 minutes more. Top with cilantro leaves and serve with chutney.