Special equipment: a grill, kitchen twine and 8 long skewers.
Split the lamb belly into two pieces, crosswise. Season both sides well with salt and let sit uncovered in the refrigerator for 24 hours.
For the cashew date romesco:
Add the olive oil, garlic and shallot to a large skillet over medium-low heat. Slowly toast the garlic and shallot, being careful not to burn, until the garlic is a light golden brown, about 5 minutes. Remove the garlic and shallot with a slotted spoon and set aside. Add the tomatoes and dates and gently heat until softened, about 5 minutes. Strain the oil and reserve.
Combine the garlic, shallot, tomatoes, dates, cashews, vinegar and 1/4 cup of water in a blender and puree. Slowly stream 1/2 cup of the reserved oil into the blender to emulsify the ingredients. Add up to 1 /2 cup more of the oil, if needed, in order to reach a smooth puree the consistency of room temperature peanut butter. As an alternative, set aside some of the ingredients before blending, then hand-chop and fold them into the romesco to create a chunky texture.
Adjust the seasoning with more salt and sherry vinegar if needed.
Prepare a grill for indirect heat, maintaining a temperature of 200 to 225 degrees F.
Brush one piece of lamb with 1/2 cup of the cashew-date romesco, roll it up as tightly as possible and secure with kitchen twine. Repeat with the second piece of lamb.
Place the lamb on the grill and frill uncovered until it reaches an internal temperature of 140 degrees F, about 1 hour. Remove and let cool to 110 degrees F. Wrap each roll tightly in plastic wrap to compress it and transfer to the refrigerator to cool completely.
For the pea and fava bean salad:
Bring a large pot of salted water to a boil. Blanch the English peas and fava beans separately in the boiling water until they are tender and bright green, about 30 seconds for the peas and 1 minute for the favas. Shock in an ice bath to stop the cooking.
Combine the English peas, fava beans, sugar snap peas, dates, lemon juice, olive oil, pickled onions, yogurt and some salt in a medium bowl. Toss well to coat evenly.
Increase the grill to high heat. Cut each roll of lamb into 6 to 8 slices, about 1 1/2-inches thick. Thread 2 slices on each skewer so that the skewer is holding the pinwheel together. Brush with some of the cashew-date romesco. Grill the lamb over high, indirect heat until the outside of the lamb is charred and some of the fat is rendered.
Brush each plate with some of the remaining cashew-date romesco; top with some pea and fava bean salad and 2 slices of lamb. Squeeze the lemon over the lamb and garnish with the mint. Reserve the remaining lamb for another use.