In a medium bowl, stir together tomatoes, corn, cumin, and chopped cilantro; set aside.
In a 2-quart microwave-safe dish with a lid, combine potatoes, shallots, and oil. Season with salt and pepper; spread in an even layer. Cover; microwave on high for 6 minutes.
Place cod in a single layer around the edge of the dish, on top of potatoes. Season with salt and pepper. Spoon tomato mixture in the center. Microwave on high until cod is cooked through and potatoes are tender, 9 to 10 minutes. Gently stir to combine; garnish with cilantro leaves.