Rinse shell-on shrimp under cold water, ensure they're clean, cut the shell along the back and remove the veins, pat dry.

Melt butter and olive oil in a saucepan, then add garlic, salt, pepper, and red-pepper flakes. Stir for about 2 minutes, then add lemon juice and zest.

Toss shrimp in half of the garlic butter mixture, cover and marinate in the fridge for 15 to 20 minutes.

Preheat grill to medium-high heat, clean grates and oil them lightly.

Thread the marinated shrimp onto skewers, leaving space between each.

Grill shrimp for about 2 minutes on each side, brushing with reserved garlic butter while grilling.

Check for doneness; shrimp should be bright pink and reach a temperature of 120°F.

Remove from grill, brush with remaining garlic butter and serve garnished with parsley.