Preheat grill or grill pan to medium-high. Fill a medium saucepan to a depth of 2 inches to a boil over high heat. Arrange potatoes in a steamer basket and set the basket in the saucepan. Cover and steam the potatoes until crisp-tender, about 5 minutes.
Meanwhile, stir together mayonnaise, garlic, vinegar and paprika in a large bowl; season with salt and pepper. Butter the cut sides of sandwich buns. Toss bell pepper with oil and season with salt and pepper.
Toss cooked potatoes with paprika mayonnaise until coated. Lightly oil grill grates; add potatoes and bell pepper to the grill. Cook potatoes and bell pepper until tender and charred, turning once, about 8 minutes. Add sandwich buns, buttered-side down, to grill to char, 2 minutes.
Rub cut side of bottom bun with cut tomato. Make sandwiches out of peppers, grilled potatoes and sliced Manchego.
Before her diagnosis, Caroline wrote a book on cakes called Cake Magic!. She started developing a birthday cake using her gluten-free mix found in that book. Check out other recipes she’s developing for her new life—and the stories behind them—on her blog, The Wright Recipes. Her next book, Catalan Food, written with chef Daniel Olivella, comes out in early September from Clarkson Potter.