Soak the frozen udon in a bowl of warm water, loosening to make sure the strands of noodles are separate. Drain and set aside. Mix together the soy sauce, dashi powder, and mirin (or sugar) with 1 tablespoon water in a small bowl. Set aside.
Heat up the oil in a skillet over medium heat. Add the sliced pork and cook until the pork is seared and cooked through, 2-4 minutes.
Add the onion and sauté, stirring, until soft, but not brown. Stir in the cabbage and carrot, cooking until soft, but still a bit crunchy, 1-2 minutes.
Add the drained udon noodles and bowl of sauce. Toss everything together, over medium-high heat, until the sauce reduces and coats the noodles, 1-2 minutes. Remove from the heat, and season with salt and pepper. Finish with sliced nori and bonito flakes, if using.