Bring a large pot of water to a boil; salt generously. Set up a large bowl of ice water nearby. Blanch the green beans until bright green, 1 to 2 minutes. Immediately, transfer the beans to the ice bath. Set aside.
Heat a large skillet over medium-high heat. Add the quinoa in a single layer and toast until beginning to brown and pop, 2 to 4 minutes. Add the pepitas, sesame seeds and sunflower seeds; toast for another 2 to 3 minutes. Remove from the heat and set aside. (The seeds can be toasted up to a week in advance.)
In a medium bowl, mix together the olive oil, balsamic vinegar, crème fraiche, honey, Dijon mustard, lemon zest and juice, and 1 teaspoon salt.
Drain the beans well and add them to the dressing. Toss with seed mixture and serve.