Preheat your oven to 400°F and line a large baking sheet with parchment paper.
Peel and cube the sweet potato, beets, and pumpkin into 1/2-inch pieces.
In a large bowl, toss the cubed vegetables with avocado oil, dried thyme, and half of the sea salt and black pepper until well coated.
Spread the vegetables in a single layer on the baking sheet and roast for 25-30 minutes, or until they are tender and slightly caramelized.
While the vegetables roast, pat the tuna fillets dry with a paper towel and season both sides with the remaining salt and pepper.
Heat a cast-iron skillet over medium-high heat until it is very hot, then sear the tuna for 1-2 minutes per side for a perfect medium-rare center.
Arrange the roasted root vegetables on a plate, top with the seared tuna, and finish with a cold dollop of cottage cheese and a bright squeeze of lemon juice.