Preheat oven to 350 degrees.
In a mixing bowl, combine 1/4 cup olive oil, garlic, and butter, and spread on bread slices, coating each side completely. Season bread with salt and pepper and place on a baking sheet.
In a small bowl, mash anchovy fillets, remaining 1 tablespoon olive oil, and a pinch of salt with a fork to create a smooth paste. Spread on top of each slice of bread.
In a small skillet over medium heat, whisk to combine soy sauce and sugar, and cook until thick and reduced, 3 to 5 minutes. Using a pastry brush, brush glaze over eel slices and place on top of each crouton. Transfer to oven and bake until crispy and slightly golden-brown, about 15 minutes. Remove croutons from oven and place on a large plate.