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Slice the sweet potato 0.5cm thick. Sprinkle with 1/2 tbsp water, cover with plastic wrap, and microwave for 3 minutes to half-cook it.
Heat perilla oil in a pan and sauté the minced garlic and cake flour over low heat for 1 minute.
Pour in the milk in 2-3 additions, stirring to avoid lumps, and simmer until thickened. Stir in the ground perilla seeds, salt, and pepper.
Layer the sweet potato in an oven-safe dish and pour over the sauce from step 3. Top with mozzarella.
Bake at 200°C for 12-15 minutes, until the top is golden.