Make the dashi: Put the mushrooms and konbu, if using, in a medium bowl and cover with 2 cups boiling water. Let steep for 15-20 minutes. Strain the liquid into a small bowl, reserving the mushrooms and konbu. Quarter the mushrooms and thinly slice the konbu, and set them aside.
Put the block of tofu on a paper-towel lined plate, top with paper towels, top with another plate, and put a weight on top to press the tofu. Let everything sit for 15 minutes or so. Dry the tofu thoroughly with paper towels, the slice into six even pieces.
Make the quick pickles: put the thinly sliced cucumber in a small bowl, toss with the salt and sugar, and then with the vinegar. Set aside.
Heat the oil in a large saucepan or medium pot (a Dutch oven would be great, here) over medium-high heat. When the oil is just beginning to smoke, add the tofu in one layer, and cook until golden brown and seared on one side, around 90 seconds. Turn and sear the other side for 90 seconds, then transfer to a plate. Add the chard stems, carrots, and parsnip, and cook, stirring, until the vegetables have some color, around 5 minutes. Add the chard leaves and cook until the leaves begin to wilt, around 2 minutes. Add the reserved dashi, the soy sauce, the mushrooms, and the sliced konbu, and cook until the parnsips and carrots are tender, around 5 more minutes. Add the tofu to the pot, nestle the pieces in, and cook until the liquid has reduced by two-thirds, around 7 minutes.
Serve over white rice, garnished with the shredded nori, if using, and scallions. Put a little pile of pickles off to the side, add soy sauce to taste, and give the whole thing a squirt of sriracha.
Notes
The nori – roasted seaweed most often seen wrapping up sushi rolls – is totally optional, but don’t omit the sriracha. Its bright spiciness elevates the dish, and snaps the deeper flavors of the mushrooms and tofu into focus.