Strain the sun-dried tomatoes, collecting the oil in a liquid measuring cup. Thinly slice the tomatoes.
Heat ¼ cup of the collected oil (if you don’t have enough, add olive oil) in a large pot or Dutch oven over medium. Add the onion and cook, stirring occasionally, until softened and translucent, about 5 minutes. Add the barley and cook, stirring occasionally, until the grains smell lightly toasted, 1 to 2 minutes.
Pour 6 cups of water into the pot. Add the sun-dried tomatoes and season with salt and pepper. Cover and bring to a boil over high heat.
Uncover, reduce the heat to medium and simmer, stirring occasionally, until almost all of the liquid has been absorbed and the barley is al dente, 30 to 35 minutes.
Reduce the heat to low and stir in the beans. Stir in the spinach, a handful or two at a time, until wilted. Taste and season with additional salt and pepper as needed.