Heat the oven to 200°F. Put a sheet pan or oven-safe plate in the oven so you have a warm place to hold the fritters between batches.

Put the grated potatoes in a colander set in the sink. Sprinkle them with 1/2 teaspoon of the salt and let them stand for 5 minutes to release some liquid.

Put the sliced leek in a medium bowl of cold water. Swish the slices with your hands, then lift the leek out of the water and pat it dry with a clean towel.

Gather the potatoes in a clean kitchen towel and squeeze hard over the sink to remove as much liquid as you can. The drier the potatoes are, the crisper the fritters will be.

Put the squeezed potatoes, dry leek, thawed peas, beaten eggs, flour, 1/2 teaspoon of the salt, and 1/4 teaspoon of the black pepper in a large bowl. Stir until the flour disappears and the mixture holds together when you press some in your hand.

Put the Greek yogurt, 1 teaspoon of the lemon zest, 1 tablespoon of the lemon juice, and a pinch of the remaining salt in a small bowl. Stir and set the bowl aside for serving.

Put the drained lentils, 1 tablespoon of the olive oil, the Dijon mustard, 1 tablespoon of the lemon juice, 2 tablespoons of water, half of the dill, the remaining 1/2 teaspoon salt, and the remaining 1/4 teaspoon black pepper in a small saucepan. Set the saucepan over medium-low heat.

Warm the lentils for 4 to 5 minutes, stirring once or twice, until they are hot all the way through and lightly glossy. Turn the heat to low to keep them warm while you cook the fritters.

Pour 2 tablespoons of the olive oil into a large nonstick skillet and set the skillet over medium heat. Let the oil heat for about 1 minute, until it looks loose and shimmery.

Scoop 4 mounds of the potato mixture into the skillet, using about 1/4 cup for each one. Flatten each mound gently with a spatula so each fritter is about 1/2 inch thick.

Cook the fritters on the first side for 4 to 5 minutes. They should look deep golden around the edges and hold together when you slide the spatula underneath.

Flip the fritters in the skillet. Cook the second side for 4 to 5 minutes, until it is browned and the center feels firm when lightly pressed.