Saute the onion, rosemary, and garlic in a fry pan, or in the bottom of your soup pot. Should have a nice aroma.
Rinse the canned beans, peas, lentils, to remove extra salt.
In a large pot or Dutch oven add equal quantities of the beans, peas, and lentils. Stir. Make sure you still have a few inches at the top -- extra beans can be reserved in the fridge, covered, for up to a week. Or make another batch right after!
Stir in the barley; stir in the onion mixture.Sprinkle generously with salt, but you may need to add more later during or after cooking. Stir in a little more oil if desired, for flavor and to prevent sticking.
Add water till an inch or two from the top. Mix thoroughly. Cover and cook over medium heat till heated thru, stirring occasionally. Lower flame to prevent boilover or burning on pot bottom if it seems imminent. Cooking for a few extra minutes (past the heated stage) on simmer will increase the flavor and the thickness, but cook it too much and the beans will fall apart. If soup seems too salty, add cut raw potato, and remove potato after cooking a few minutes. Potato should draw out salt.
Cool slightly and serve, add salt if needed. Goes well with crusty Italian vegan bread, warmed or toasted. If the soup is extra-thick, you can spread it on the vegan bread!
Extra soup can be frozen in individual servings and heated in the microwave. Do not add extra salt before freezing -- add more if needed when soup is reheated.