Peel and dice the russet potatoes into approximately ½-inch cubes, then boil them in a large pot of water for 12-15 minutes until fork-tender. Drain and set aside.

In the same pot, cook the frozen peas for 3-5 minutes until heated through, then drain.

Melt the unsalted butter in a medium saucepan over medium heat, add the diced onions, and sauté for about 5-7 minutes until translucent.

Pour in the heavy cream, stir to combine, and bring to a simmer for about 2-3 minutes until the sauce thickens slightly.

Fold the cooked potatoes and peas into the creamy sauce, season with dill, salt, and pepper, and mix until well-combined.

Let the dish cool slightly before serving warm, garnished with fresh dill if desired.