Bring carrots, ½ cup water, and instant dashi granules to a boil in a small saucepan over medium. Decrease heat to medium-low to maintain a simmer; cover and cook until carrots are fork-tender, about 5 minutes. (Most of the water will have evaporated.) Add onion; cook, stirring constantly, until just softened, 1 to 2 minutes. Remove from heat, and let cool 5 minutes.

Transfer carrot mixture to a blender; add oil, ginger, miso, vinegar, agave syrup, and remaining ½ cup water. Process until smooth, about 30 seconds. Season with additional instant dashi granules or salt to taste, if desired. Make the salad

Cook noodles in lightly salted water according to package directions. Drain noodles, and return to pot; add cold water to cover by 1 inch. Swish noodles around in pot to quickly cool; drain well. Let stand until noodles are room temperature, 15 minutes.

Divide noodles evenly among 4 shallow bowls. Arrange radishes, cucumbers, tomatoes, and dressing evenly over noodles. Sprinkle evenly with chopped cilantro and toasted seaweed pieces. Garnish with cilantro leaves. Serve immediately, using a spoon and chopsticks to stir everything together.