Prepare the dashi stock and let it cool to room temperature. If using instant dashi, dissolve the powder in warm water and cool completely.

In a mixing bowl, gently beat the eggs without creating too many bubbles. Add the cooled dashi, soy sauce, mirin, and salt, then stir gently to combine.

Strain the egg mixture through a fine-mesh sieve into a measuring cup or bowl to ensure a silky smooth texture.

Divide the shiitake mushroom slices, kamaboko (if using), and spinach leaves evenly among 4 heatproof cups or chawanmushi bowls.

Place 2 shrimp into each cup, positioning them attractively near the sides.

Carefully pour the strained egg mixture into each cup, filling to about 80% capacity. Remove any surface bubbles with a spoon.

Cover each cup tightly with aluminum foil to prevent condensation from dripping onto the custard.

Place the cups in a steamer over boiling water. Reduce heat to medium-low and steam for 20-25 minutes until the custard is set but still jiggles slightly in the center. The internal temperature should reach at least 72°C to ensure the eggs and shrimp are safely cooked.

Carefully remove the cups from the steamer. Garnish each with a thin slice of lemon zest and serve immediately while warm.