Step 1
Partially fill a large bowl with cool water, add a handful of kosher salt, and swish with your hand to dissolve. Add 1 lb. small clams, preferably littleneck (10–12), scrubbed; let sit 15 minutes to expel any grit. Carefully drain and rinse. If clams expelled a lot of grit into the bowl, repeat with a fresh bowl of cool water and salt. Set aside.
Step 2
Bring one 5×3″ piece dried kombu and 6 cups water to a boil in a medium Dutch oven or other heavy pot over medium-high heat and cook until water starts to turn pale yellow and smells sealike, about 5 minutes. Add 2 large or 4 small zucchini (14–16 oz.), halved, thinly sliced into half-moons, 1 small onion, cut in half through root end, thinly sliced, and 5 garlic cloves, coarsely chopped, and cook, stirring occasionally, until onion is translucent and zucchini are nearly cooked through, 6–8 minutes.
Step 3
Using a large spoon, scoop large dollops of one 11-oz. tube or 14-oz. package silken tofu into broth, then add one 3.5-oz. package enoki mushrooms or 5–6 oz. beech mushrooms, stems trimmed, and reserved clams and press everything down gently to submerge in liquid. Cover pan and cook until clams open, about 5 minutes, depending on size of clams. (If any clams do not open, cover, and continue to cook, checking every minute or so, until open; discard any that do not open after 5 minutes more.) Try to avoid stirring so tofu doesn’t break up.
Step 4
Reduce heat to low and skim any foam from surface. Add 2 Tbsp. fish sauce, 1 Tbsp. soy sauce, and 1 tsp. Diamond Crystal or ½ tsp. Morton salt; taste and season with more salt if needed. Remove pot from heat and slowly drizzle 1 large egg, beaten to blend, all over the stew’s surface. Scatter 2 scallions, chopped, over, then drizzle in ½ tsp. toasted sesame oil.
Step 5
Cover stew and serve, keeping covered until you are ready to ladle into bowls, with steamed rice alongside.
