In a large pot, combine the kombu and 8 cups water. Let sit until the kombu is soft, 15 to 20 minutes.
Place the pot over medium heat just until small bubbles form (you don’t want the mixture to boil yet). Remove the kombu, then add the bonito flakes and stir to combine.
Bring the mixture to a boil, then turn off the heat and let sit for 2 minutes. Using a strainer or slotted spoon, strain the bonito from the dashi and discard the bonito.
Add the squash and jalapeño to the dashi and bring to a boil. Then reduce the heat and simmer until the squash is tender, about 10 minutes.
Divide the dashi, squash, and jalapeño among 4 serving bowls. Cut the tofu into four pieces and add one piece to each bowl. Top with watercress and more bonito flakes and season with salt and pepper.
Photography and prop styling by Julia Gartland. Food styling by Samantha Seneviratne.