Adjust the oven rack to the second position inches from the top (about 6-8 inches from the flame) and preheat the broiler on high. Spread the cherry tomatoes, onion and garlic on a small sheet pan and drizzle with 1 tablespoon olive oil. Place the sheet pan in the oven and broil until the vegetables achieve a subtle char, about 7-9 minutes. Every broiler is slightly different, so keep an eye on them and turn the baking sheet as needed. Remove from oven and set aside to cool slightly.
Preheat oven to 425 degree F and prepare a large sheet pan with parchment paper.
Slice potatoes in half lengthwise (or leave whole if using small baby potatoes).
Place on prepared sheet pan, drizzle with 1 tablespoon olive oil and toss to coat. Spread in a single layer and sprinkle with salt and pepper to taste. Roast in the preheated oven 25-30 minutes or until fork-tender and crsipy-golden.
Meanwhile make Romesco Sauce: Place the roasted tomatoes, onion, and garlic, along with all the other romesco ingredients in a food processor. Pulse until well combined.Taste and adjust salt and spiciness.
Place potatoes on a platter and serve immediately alongside romesco sauce.