Whisk together all of the dressing ingredients in a glass or plastic measuring cup. Cover and chill for 10 minutes or overnight to plump up the chia seeds.
Pour the quinoa, beans, and fresh herbs into a large mixing bowl. Keep this mixture chilled while you grill the remaining ingredients.
Preheat a cast-iron grill pan over medium-high heat, or heat an outdoor grill as per manufacturer's directions. Rub the fruit and veggies with a thin coating of olive oil and sprinkle with a pinch of salt. Grill the onions until browned, about 3 minutes; set aside. Grill the peaches on each side for about 2 minutes, until the outside is hot and slightly charred; set aside. Grill the ears of corn until the kernels are lightly charred, turning the cobs occasionally, 3 to 4 minutes. Cool the vegetables and fruit just enough to be handled safely, then dice into bite-sized pieces. Use a knife with a thin blade to slice the corn kernels off the cobs.
Transfer the chopped fruit and veggies to the bowl with the quinoa, add the pumpkin seeds, and then pour on the dressing and toss thoroughly. Serve at room temperature or chilled.