Preheat the oven to 190°C (375°F) and line a shallow roasting pan with baking paper. Wrap the halved garlic head in foil to prevent it from burning. Arrange the halved tomatoes on the tray, drizzle with a touch of olive oil, and sprinkle with paprika, salt, and black pepper. Add the red capsicum and the wrapped garlic to the tray.
Roast everything for 25–30 minutes until the tomatoes soften and the capsicum skin blisters on all sides. Turn the capsicum halfway through roasting to ensure it chars evenly. The garlic should be soft and fragrant — if not, return it to the oven for a few more minutes.
Once roasted, transfer the hot capsicum to a bowl, cover it with a clean tea towel, and allow it to steam for about 10 minutes. This loosens the skin, making it easier to peel. Remove and discard the skin and seeds once cool enough to handle. Gently squeeze the garlic cloves from their skins and set aside.
Meanwhile, heat a small amount of olive oil in a frying pan over medium heat. Toast the sourdough slice on both sides until golden and crisp. In a food processor, combine the roasted tomatoes, peeled capsicum, garlic cloves, toasted bread, almonds, vinegar, paprika, and chilli flakes. Blend the mixture while slowly drizzling in the olive oil until you achieve a coarse, textured sauce. Taste and adjust seasoning with salt and pepper as needed. The Romesco sauce should remain slightly chunky for added depth and character.
Set aside to serve warm, or refrigerate in a sealed container for later use.