Cook bell peppers in olive oil on medium heat. Season generously with salt and pepper. Add onion and garlic. Stir in olives, then tomatoes, stirring constantly. Add serrano and potato. Reduce heat to low and add herbs.
While vegetables are cooking, beat together egg whites, salt (to taste), and water, until mixture forms stiff, but not dry, peaks. Separately combine cream and yolks, and thoroughly fold into egg whites.
Melt butter in a large, ovenproof pan over high heat. Pour out half of butter and reserve. Add 3/4 of egg mixture to pan, then add vegetables, distributed evenly. When underside is cooked (takes less than a minute), pour on remaining egg mixture. Place pan in broiler for 2 minutes.
To serve, invert onto a heated serving platter and brush with reserved melted butter.