Place the soaked and drained pinto beans in a heavy-bottomed pot and cover with the dashi stock.
Bring the liquid to a boil over medium-high heat, then reduce to a low simmer.
Skim off any foam or impurities that rise to the surface using a fine-mesh skimmer.
Add the ginger slices, sake, and sugar to the pot. Cover with an otoshibuta (drop lid) or a parchment paper circle to ensure even heat distribution.
Simmer for 45 minutes or until the beans begin to soften.
Incorporate the diced carrots, sliced shiitake mushrooms, mirin, and soy sauce.
Continue simmering for another 30 to 40 minutes until the beans are tender and the liquid has reduced by approximately half.
Drizzle with sesame oil and remove from heat.
Allow the stew to sit for 10 minutes before serving to allow the flavors to penetrate the beans deeply.