Blend all aioli ingredients in a food chopper, or blender, and pulse until smooth. Chill in the fridge until ready to serve.
Preheat grill to 350°F.
Cut romanesco in half (a flat surface will make it easier to grill).Brush romanesco with olive oil and season with salt and pepper. Grill romanesco until evenly charred, flipping as needed. Drizzle with herb aioli and serve hot.