Preheat your oven to 425°F (220°C).
On a large baking sheet, toss the sweet potato, parsnip, and beet cubes with 2 tablespoons of olive oil, 1 teaspoon of kosher salt, ½ teaspoon of black pepper, and the dried thyme until evenly coated.
Spread the vegetables in a single layer on the baking sheet and roast for 25-35 minutes, tossing halfway through, until tender and caramelized at the edges.
While the vegetables roast, pat the sirloin steak dry and season both sides generously with salt and pepper.
Heat 1 tablespoon of olive oil in a cast-iron or heavy-bottomed skillet over medium-high heat.
Sear the steak for 4-6 minutes per side for medium-rare, or until it reaches your desired doneness.
Transfer the steak to a cutting board and let it rest for at least 10 minutes.
While the steak rests, make the vinaigrette by whisking together the ¼ cup olive oil, lemon juice, Dijon mustard, and honey in a small bowl; season with a pinch of salt and pepper.
Slice the rested steak thinly against the grain.
To assemble the bowls, divide the cooked quinoa among four bowls.
Top with the roasted root vegetables and sliced beef.
Garnish with crumbled feta cheese and fresh parsley.
Drizzle with the simple vinaigrette just before serving.