Start the barley: Put a medium saucepan on the stove, add 3 cups of water, and bring it to a boil over high heat.

Rinse the pearl barley under cold water in a fine-mesh sieve, then let it drain well.

Add the barley and 1 teaspoon of the kosher salt to the boiling water in the saucepan. Stir once, lower the heat to medium-low so the water bubbles gently, and cook for 28 to 32 minutes, until the grains are tender with a little chew left in the center.

Brown the mushrooms: While the barley cooks, set a large skillet over medium-high heat and add 2 tablespoons of the olive oil.

Put the sliced mushrooms into the skillet and spread them into as even a layer as you can. Leave them alone for 4 minutes so they can brown on the bottom.

Stir the mushrooms, add 1/4 teaspoon of the kosher salt and a few grinds of black pepper, and cook for 4 to 5 minutes more, until the mushrooms are browned and the skillet looks mostly dry.

Transfer the browned mushrooms from the skillet to a plate and set them aside.

Build the skillet: Lower the heat under the same skillet to medium and add the remaining 2 tablespoons olive oil.

Add the sliced shallot to the skillet and cook for 2 to 3 minutes, stirring now and then, until it softens.

Add the sliced garlic to the skillet and cook for about 30 seconds, stirring, just until it smells fragrant.

When the barley is ready, scoop 1 cup of the hot cooking water from the saucepan into a heatproof cup. Drain the barley in a colander.

Add the drained cannellini beans and 1/2 cup of the reserved barley cooking water to the skillet with the shallot and garlic. Cook over medium heat for about 2 minutes, stirring gently, until the beans are hot.