1. Preheat oven to 400°F. In a medium mixing bowl, combine 1 tablespoon olive oil, the mustard, 1/4 teaspoon thyme, a pinch of salt, freshly ground pepper, and the juice of half the lemon.
2. Mix together, then add the onions and toss well.
3. Spread on a baking sheet and bake for 12-15 minutes, until the onions are just brown around the edges.
4. Remove from the oven, and raise the heat to 450°F.
5. While the onions are cooking, rinse out the mixing bowl. In the same bowl, combine the zest of the lemon, the red pepper flakes, the remaining lemon juice, and 2 tablespoons olive oil.
6. Add a pinch of salt and the shrimp, and toss to distribute.
7. Scoop the shrimp and all its oil on top of the onions. Return to the oven and cook 5 minutes, until the shrimp are pink and cooked through.
8. Serve immediately. You can eat these shrimp plain, or pile them on top of rice or quinoa.