Heat a frying pan and melt butter over medium heat.
Add the mushrooms, onion and beef and sprinkle little salt.
Saute for about 1 minute each side until brown.
Add the soy sauce, mirin, and sugar (if adding). Stir well.
Add water then bring to boil.
Add udon and cook for 1 minute and 30 seconds (*Before adding udon, you could boil udon in a boiling water and rinse under running water to get rid of acidity).
Add the tomatoes and cook for 30 seconds.
Transfer to a serving bowl. Garnish with spring onion.