Gently clean the fresh uni pieces under cold water, and set aside on a paper towel to drain.

In a large skillet, melt the unsalted butter over medium heat.

Add the finely diced onion and sauté for about 5-7 minutes until soft and translucent.

Stir in the minced garlic and cook for about 1 minute until fragrant.

Reduce heat to low, pour in the half and half, and add the Japanese mayo.

Season with salt, black pepper, and chicken bouillon; stir gently until combined.

While the sauce simmers, cook the udon noodles according to the package instructions.

Drain the udon and add to the skillet, tossing until noodles are coated.

Gently fold in the uni and optional masago, heating for 1-2 minutes.

Serve immediately, garnishing with extra masago if desired.