3/4cupunsalted butter, at room temperature, cut into 6 equal pieces
salt and white pepper
1tablespoon finely chopped chives
1 medium plum tomato, peeled, seeded and diced
Half an hour before serving, preheat the oven to 350°F. When you are ready to finish the dish, bring the vegetable nage to a boil in a medium saucepan over medium heat, then reduce the heat to low. Add 2 pieces of butter and stir until melted, then pour the nage into a blender. Blend at high speed, removing the lid to add the remaining butter a piece at a time, and blend just until emulsified. Season to taste with salt and white pepper, pour into a glass measuring cup, and skim off any foam that rises to the surface. (Use within half an hour; to reheat, if cold, bring the sauce back to a simmer and blend again for 10 to 15 seconds to reemulsify.) Stir in the chives and diced tomato.
Place the lasagna in the oven and reheat until it is warmed through, about 4 to 6 minutes. Using a flat-ended metal spatula, transfer the lasagna to individual heated appetizer plates. Spoon a little of the sauce over and around each one.