Slice the beef thinly in bite sized pieces and combine it with the soy sauce and baking soda in a bowl for 10 minutes.
Cook the udon noodles according to the package instructions while the beef marinades and set aside.
Heat a dutch oven on high and add the beef to sear it on all sides (about 3-5 minutes total). Remove and set aside.
Reduce the heat of the pot to medium and add the cooking oil. Add the onions, garlic and shiitake mushroom and saute for 2-3 minutes, until softened.
Add the beef back to the pot and pour in the dashi broth. Bring to a boil.
Add the diced potatoes and cook (simmer) until soft, about 10-15 minutes.
Add the curry paste and stir until it dissolves and the broth is thickened. Add additional salt to taste.
Place the udon noodles in a bowl and pour the curry on top. Garnish with sliced green onions and fried garlic.
Notes
Freeze the beef for 30 minutes before slicing it to make it easier to cut thin strips.
The beef should be seared over high heat to get a nice char on the outside. This adds to the flavor.
Cook the noodles while the beef is marinading so they are ready when the curry is done.
Dice the potatoes in ½" pieces so they cook quicker and are easier to eat.
If you're using dashi powder, make the stock while the noodles are cooking according to the package instructions.
Substitute dry shiitakes for fresh if you can't find fresh. Soak the mushrooms in warm water for 15 minutes until they are softened. Remove the stems and roughly chop them.