In a high-speed blender, add all of the ingredients for the cauliflower sauce (from broth to ground black pepper). Blend until well combined. Set aside in a bowl.
Preheat oven to 400°F. Line 2 baking sheets with parchment paper or lightly greased aluminum foil.
Prepare cauliflower: Cut off the outer green leaves of the cauliflower. Carefully trim any excess stem but be sure to keep the core intact because it helps hold the cauliflower steaks together. Rinse the cauliflower and gently pat dry. Place cauliflower on a cutting board, stem side down. Using a sharp, large knife, slice the cauliflower head right down the center. Working with each half, cut ½-1 inch thick slices from the center. Repeat working outwards from the head’s center until sliced slabs no longer hold together. Depending upon how large the cauliflower is, it should yield 2-4 large pieces. You can still roast the florets that fall apart as they taste just as good!
Arrange the cauliflower slices on the prepared baking sheets, being sure not to overcrowd the pans. Using a pastry brush, evenly brush the cauliflower sauce mixture over the cauliflower until well coated. Place in the oven and bake for 40-45 minutes, until fork tender and the bottom is browned. Brush more sauce about half way through the cooking time.
Prepare the beans: as the cauliflower is roasting, prepare the butter beans. Heat a large skillet over medium high heat. Once the skillet is hot, add the tablespoon of olive oil. Add the shallots and cook for 3-5 minutes until translucent. Add the garlic (and chili flakes if using) and stir for 1-2 minutes until fragrant. Next add the butter beans and ½ cup vegetable or chicken broth and turn the heat down to a simmer. Cook for 5 minutes, broth should start to get absorbed. Season with salt and pepper to taste. Turn the heat to low and add sherry vinegar and stir to combine. When cauliflower is almost done, remove beans from heat and stir in the fresh herbs. Taste and adjust seasoning.
In a small bowl mix pesto and broth to make the pesto sauce for drizzling. Set aside.
When ready to eat, plate the meal by serving the cauliflower over the bean mixture and drizzle with pesto sauce. Enjoy!