Start by preparing the brussels sprouts. Wash the sprouts and cut off the stems. It's okay if some of the leaves come off- just set them aside!
Next, bring a pot of salted water to a boil.
Once the water is boiling, add the brussels sprouts and boil for 15 minutes.
Once the sprouts are done boiling, pop them in an ice bath for 5-10 minutes, until they have cooled. Then, drain the water from them and dry.
Next, preheat oven to 450 F and line a baking sheet with parchment paper.
Spread the brussels sprouts out evenly on the baking sheet.
Using the back of a glass or a plate, smash the brussels sprouts about 1/2 inch thick.
Brush each one with olive oil, then sprinkle with a generous amount of sea salt and black pepper.
Place the tray in the oven then bake for 35 minutes, making sure to flip them at the halfway mark.
While the sprouts are in the oven, prepare the romesco sauce. Add all of the sauce ingredients to a blender or food processor, and blend/process everything until it's mostly smooth and creamy, but leaving some texture. Taste the sauce and adjust the seasonings as needed.
Finally, once the Brussels sprouts are done, remove them from the oven and drizzle the dressing on top, or serve it in a small dipping bowl alongside them. Serve immediately, and enjoy!