Preheat the oven to 400°F.
On a parchment-lined baking sheet, toss the Brussels together with 1 tablespoon of olive oil and a sprinkle of salt. Bake for 20-30 minutes until they are browned but still have a bite to them.
In the meantime, bring a pot of water to boil. Cook the soba noodles according to package instructions until tender (about 8 minutes for the 100 percent buckwheat version). Reserve 1/4 cup of cooking liquid. Drain the noodles and rinse them under warm water until some of the starch is gone.
Add the miso sauce to a mixing bowl and dilute with some of the cooking water until the sauce is light and creamy – more of a sauce than a paste. Toss together with the noodles.
To serve, divide the noodles between two bowls and top with a generous handful of Brussels sprouts and chard stems (if using).