1/4 cup buckwheat groats
1 1/2 pounds yellow summer squash, julienned on a mandoline, or with a knife
2 scallions, thinly sliced
1/4 cup coarsely chopped fresh mint
1 teaspoon coarsely chopped fresh marjoram or oregano
3 tablespoons olive oil
1 tablespoon fresh lemon juice
Freshly ground black pepper
4 ounces feta, thinly sliced
Toast buckwheat in a large, dry skillet over medium-high heat, tossing often, until fragrant, about 4 minutes. Transfer to a plate; let cool.
Toss squash, scallions, mint, marjoram, oil, and lemon juice in a large bowl; season with salt, pepper, and more lemon juice, if desired. Add feta and toasted buckwheat and toss gently to combine.
Do ahead: Buckwheat can be toasted 2 days ahead. Store airtight at room temperature.