Step 1

Bring a large pot of water to a boil.

Step 2

Meanwhile, combine bean cooking liquid and beans in a microwave-safe container and microwave on high until steaming, 1 to 2 minutes (timing will depend on strength of your microwave). Alternatively, combine bean cooking liquid and beans in a small saucepan, set over medium-high heat, and cook until both liquid and beans are steaming hot, 3 to 4 minutes.

Step 3

Transfer hot bean mixture to a blender jar, an immersion blender cup, or another tall-sided container that just fits the head of your immersion blender. Add fermented bean paste, oil, and vinegar, and blend until completely smooth, about 1 minute. Season with salt to taste, transfer 1/2 cup (120ml) of sauce to large mixing bowl, and set remaining sauce aside.

Step 4

Add one portion of noodles to boiling water, and cook according to package directions or recipe directions.

Step 5

When noodles are cooked, drain briefly in strainer or noodle basket, then add to bowl with sauce. Using tongs or chopsticks, immediately toss and stir noodles vigorously in the liquid at the bottom of the bowl, until the noodles are well-coated in sauce, about 30 seconds. Taste and adjust seasoning with more vinegar and salt to taste.

Step 6

Transfer dressed noodles to an individual serving bowl, and using a rubber spatula, scrape any remaining sauce over top. Garnish with sliced scallions and onsen egg (if using). (Other toppings, like leftover sautéed or roasted vegetables; poached, roasted, or pan-seared meats; or even some additional cooked beans, can be added as they are available or desired, too.) Serve immediately.

Step 7

Repeat steps 4 through 6 with remaining portions of noodles and sauce, wiping out the mixing bowl between each round, and dressing each portion of noodles with 1/2 cup (120ml) of sauce.