While garlic is roasting, trim the cauliflower leaves and the extra bit of stalk at the bottom. Cut the head right down the middle, from top to bottom. From there, cut each half into 4 wedges. In a medium bowl, toss the cauliflower wedges with 2 tablespoons olive oil, salt, and pepper. Remove the pre-heated baking sheet from the oven and carefully lay the cauliflower wedges onto the surface, leaving some space around each piece. You should hear sizzling. Roast cauliflower until tender and well-browned, about 25 minutes (I flipped mine halfway).

Once slightly cooled, transfer cauliflower to a cutting board and cut into bite-size pieces. Unwrap roasted garlic and squeeze cloves out into a small bowl. Mash roasted garlic with a fork to make a smooth puree. Whisk in the lemon juice, pepper flakes, some salt + pepper, and 1/4 cup of olive oil. Set aside.

Cook brown rice pasta according to package directions with a fat pinch of salt in the water. Reserve 1 cup of pasta cooking water before draining. Once drained, return cooked pasta to the pot. Add chopped cauliflower, garlic sauce, arugula, extra lemon juice to taste, and 1/4 cup reserved cooking water. Add more cooking water as necessary to achieve desired consistency. Serve pasta with handfuls of chopped walnuts on top and extra chili flakes.