Pastel de Papas (Chilean Potato Pie)

While the potatoes cook, heat the oil in a large skillet over medium heat. Add the onion and cook, stirring occasionally, until it turns translucent, about 5 minutes. Stir in the garlic for the last minute of cooking.

Add the diced tomatoes and tomato paste to the skillet. Continue to cook, stirring, until the tomatoes soften and start to break down, which should take 3 to 5 minutes.

Drain the cooked potatoes in a colander, then let them steam dry for 1 to 2 minutes to remove excess moisture. Return the potatoes to the pot, add the butter and salt, and mash them until completely smooth.

Allow the mashed potatoes to cool until they are just warm to the touch, about 5 to 10 minutes. Then, stir in the beaten egg until fully incorporated.