In your smallest saucepan, bring the remaining 3⁄4 cup water and the rice flour to a boil over medium-high heat, whisking constantly.
Reduce the heat to medium and continue whisking until the mixture thickens, about 2 minutes. Remove from the heat and set aside to cool to room temperature and thicken further.
While the rice paste is cooling, trim the leek and slice it into 1∕8-inch or thinner rings. Place the rings in a small bowl and cover with water. Set aside.
Remove the stems from the collards and lay the leaves on top of each other. Roll them up and cut into 1⁄2-inch strips. Place the strips in the bowl with the pumpkin.
Once the rice paste is cool, place it in the bowl of your food processor along with the garlic, ginger, gochugaru, red pepper flakes, fish sauce, onion, and carrot.
Process until the mixture is relatively smooth.
Remove the leek from the soaking liquid, being careful not to stir up any grit at the bottom of the bowl, and place it in the bowl with the pumpkin and collard pieces.
Put on your kimchi gloves. Scoop the blended rice paste into the bowl with the vegetables and use your hands to mix the ingredients together and to coat the vegetables. Once the ingredients are well mixed, remove the glove from your nondominant hand and use that hand to hold a clean quart jar.
Pack the mixture in the bowl tightly into the jar, leaving an inch of headspace at the top of the jar. Wipe the rim of the jar with a clean, damp cloth to remove any paste residue.
Cover your jar loosely with its lid, and let it sit at room temperature to ferment for 6 days to 3 weeks. When it tastes sour enough for you, secure the lid and move the jar to the fridge for storage.
Enjoy chilled or room temperature as a side dish.