When cooking brown beans, the longer they stay on the heat and cook, the softer they become. If the beans become soft enough, you can use that to your advantage to help thicken the liquid into a soup consistency. If you don't want to cook the beans any longer, and the liquid doesn't seem as thick as you would like, you can add a kitchen staple to the mixture to remedy the situation.
Place the bottom of your spoon up against three or four of the softened, cooked brown beans in the pot while it's still on the heat.
Mash the beans with the bottom of the spoon against the side of the pot.
Stir the mashed beans into the rest of the mixture in the pot to thicken the liquid.
Repeat Steps 1 to 3 two or three times to gain the consistency you desire.
Pour 2 tbsp. of cold water into a small container and add 1 tbsp. of cornstarch. Stir the cornstarch and water to mix until smooth.
Dip a ladle of hot bean juice out of the pot full of brown beans and pour it into a bowl. Measure 2 tbsp. of juice and pour it into the the cornstarch and water mixture. Stir to mix well.
Add the water, cornstarch and bean juice mixture to the pot, gradually, while stirring constantly, until you notice that the bean juice has formed a thick soup.
If you're worried about altering the taste of the beans, forgo the cornstarch and use the mashing method.
Be careful when transferring the hot bean juice to avoid burns.