Bring a pot of water to a boil and cook the udon according to package instructions. Drain and set aside.
Slice open the mentaiko sacs and scrape out the roe with a spoon or knife. Discard the thin membrane. Set aside.
In a pan over medium-low heat, melt the butter.
Add the minced garlic and sauté until fragrant, about 20–30 seconds.
Lower the heat and pour in the heavy cream, soy sauce, and mirin. Stir to combine.
Add the Kewpie mayo and mix until smooth and creamy.
Stir in about 70% of the mentaiko, reserving the rest for topping. Keep the heat low so the roe stays tender and doesn’t fully cook.
Add the cooked udon to the sauce and gently toss until the noodles are evenly coated and glossy.
Transfer to plates.
Top with the remaining mentaiko, sliced green onions, and a soft-boiled egg.
Serve immediately and enjoy while warm.