Place the roasted red pepper, bread, garlic and walnuts in a food processor work bowl and process for about 1 minute, scraping down sides once halfway.
Add the remaining ingredients and process until completely smooth, another minute or two. Taste and season with additional salt and lemon juice if needed.
Garnish with pistachios, a swirl of olive oil and a dusting of paprika, if desired.
To roast a red bell pepper, place under a broiler, on a hot preheated grill or directly on a gas flame on the stovetop. Roast the pepper, rotating it as it turns black on all sides. Place the pepper in a bowl and lightly cover to steam as it cools. When cool enough to handle, peel off the skin and seeds and discard.
Substitute 3/4 cup jarred, fire-roasted red peppers (drained) in place of the freshly roasted pepper.
Toast walnuts for 10 minutes in a 350-degree oven.