Heat the oven to 375°F (190°C).
Put the barley, water, bay leaf, dried chile, and 1 teaspoon salt in a small saucepan with a lid. Bring to a boil, then quickly reduce the heat to a simmer. Cover and cook until the barley is tender and all the water has been absorbed, 45 to 60 minutes for hulled barley, about 30 minutes for pearled barley. If the barley is tender but there’s liquid left, just drain it off; if the water has been absorbed but the barley isn’t fully tender, add a few tablespoons more water and keep cooking until tender.
When the barley is ready, drain well and discard the bay leaf and chile. Toss the barley with a nice glug of olive oil and spread onto a tray so the grains are separate. Cool completely at room temperature.
Meanwhile, arrange the beets in a baking dish in a single layer. Season with salt and pour ¼ cup (60 ml) water into the dish. Cover tightly with foil, transfer to the oven, and steam-roast until the beets are tender when pierced with a knife. Depending on the size and density of the beets, this could take between 30 minutes and 1 hour.
When the beets are tender, let them cool until you can handle them, then rub or pare away the skins. Cut the beets into bite-size chunks, pile into a bowl, and toss with the vinegar, ½ teaspoon salt, and many twists of black pepper. Let the beets sit a minute to absorb the vinegar, then drizzle on some olive oil and toss again.
Cut the plums into the same size chunks as the beets and add to the bowl, along with the scallions, pistachios, barley, and a pinch of chile flakes.
Toss everything again gently, so you don’t smash the plums. Taste and season with more vinegar, salt, black pepper, chile flakes, or olive oil —you want to get a nice sweet/salty/spicy thing going. Shower with the fresh herbs and serve right away, at cool room temperature.