Place the oven racks in the upper and lower thirds of the oven and preheat to 425°F (220°C). Line 2 large baking sheets with parchment paper.
Cut the potatoes lengthwise into quarters, then slice each quarter in half to make 8 equal-sized wedges. Transfer the potatoes to one of the prepared baking sheets. Drizzle the potatoes with the olive oil and toss to coat, then spread out in an even layer and sprinkle with the salt. Bake on the lower rack until the fries are golden brown and crispy, 40 to 45 minutes, flipping halfway through. Season with more salt, if needed.
Meanwhile, start the Fritters. Rinse the frozen peas in a colander under warm running water until defrosted, about 30 seconds, then let sit in the sink to drain. In a large bowl, combine the cooked quinoa, mozzarella, spinach, scallions, pepper and salt. Using your fingertips, lightly squish the peas as you add them to the bowl. Pour in the eggs and stir until well mixed. Sprinkle in the breadcrumbs and stir to mix.
Pack the mixture into a ¼-cup measuring cup. Turn the measuring cup over on the other prepared baking sheet and shake it to release the mixture (see note #3). Repeat with the remaining mixture, spacing the fritters evenly on the baking sheet. (You should have 15 to 18 fritters.)
Using damp fingertips, smooth out the shape of the fritters and tuck in any loose spinach. Bake on the upper rack until golden brown and firm to the touch, about 30 minutes.
In a small bowl, stir together the yogurt, pickle, dill, capers, lemon juice, pepper and salt. Taste and adjust seasonings, if needed.
Divide the hot fries and pea fritters among plates and serve with a generous scoop or two of tartar sauce.