Preheat the oven to 400°F.
Roast the walnuts in the oven until fragrant and starting to color, between 8 and 10 minutes. Drizzle with a little olive oil and sprinkle with a pinch of sea salt, then toss to combine. Set the walnuts to one side to cool.
In a large bowl, combine the beets, lentils, shallot, three-quarters of the feta, the parsley, lemon zest and juice, vinegar, and a good drizzle of the remaining olive oil. Toss until well combined.
Pile the mixture onto a large platter, scatter with the walnuts, crumble the rest of the feta over the top, and give it one last drizzle of olive oil and a good grind of black pepper.
Serve with warm flatbreads or toasted sourdough.
