Scrub the beets and place in a large pot. Cover with water by 1 to 2 inches. Bring to a boil and boil until tender enough to slide a knife into the center with ease, about 45 minutes. If the water drops below the beets, add more and bring to a boil again. Once tender, drain and let the beets cool enough to handle then cut into quarters and set aside.

While the beets are cooking, make the walnuts.

Transfer to a baking sheet and cook for 20 minutes, stirring after 10 minutes.

Once baked, remove the walnuts from the baking sheet so that they stop cooking and set aside.

Cook the pasta in a large pot of lightly salted boiling water until al dente.

While the pasta is cooking, heat a small skillet over medium-high heat. Once heated add the 1 TBSP of butter and 1 TBSP olive oil. Once the butter is melted add the red onions and a generous pinch of salt.

Cook the onions for about 4 minutes, stirring regularly, then add 1 sprig of rosemary to the skillet and continue to cook for an additional 3-4 minutes until the onions have cooked down and are caramelized in spots. Discard the rosemary sprig.

When the pasta is al dente, scoop out 1 cup of pasta cooking water and set aside. Drain the pasta.

Add ¼ cup of the reserved pasta cooking water to the blender along with the quartered cooked beets, cooked red onions, Cotija, Greek yogurt, olive oil, lemon juice, and 1 tsp salt and run until smooth. At this point if the sauce is too thick you can blend in more pasta cooking water 1-2 TBSP at a time, blending between each addition, until the desired consistency is achieved.

Transfer the pasta to a serving bowl. Pour the sauce over and toss well to coat.

Scatter the fresh herbs, finely chopped walnuts, Cotija cheese crumbles, and lemon zest. Serve immediately.