Season all sides of catfish with salt and pepper. Mix together shredded Russet potatoes and sweet potatoes. Using both hands, place potatoes in palms of hands and squeeze water out of potatoes. Add onions to potatoes. Mix well. Season mixture with salt and pepper. Brush fish with beaten egg. Spread potato mixture on top of fish. Place oil in a medium saute pan. Place pan on medium heat. When pan is hot, place fish with potato side down in pan. Cook fish until potato side is brown. Turn fish to other side. Cook on the other side for two minutes. Remove fish and saute pan from heat. Place fish in baking pan. Cook in a 350 degree oven for 12 minutes. Return saute pan to medium heat. Add fish stock, mustard, and vinegar. Stir mixture. Cook about 4 minutes. Strain and serve with fish. Pour half a portion of sauce on bottom of plate. Lay half a portion of fish on top of sauce. Do the same with other half portion of sauce and fish. Garnish with lemon and lime slices.