Roast the red bell peppers on a cast iron plate until chard all over. Transfer to a bowl and cover tightly with plastic wrap. Wait until cool enough to handle to discard the charred skins and the seeds.

Transfer the red bell peppers to the bowl of a food processor together with the basil, walnuts and olive oil. Process until smooth to your liking. Transfer to a bowl and add the grated garlic and lemon juice. Stir to combine and season to taste with the sea salt.