In a large bowl, I whisk together the olive oil, minced garlic, lemon zest, lemon juice, tarragon, mustard, and salt. This dressing does double duty: it flavors the filling and keeps everything moist while cooking. I like to whisk it until it looks slightly thickened and well combined.
Next, I add the cooked quinoa, chopped shrimp, and green onion straight into the dressing. I toss everything gently so the shrimp doesn't break apart, making sure every grain of quinoa gets coated. This is the point where I usually taste a spoonful to check the seasoning.
I lay the pepper halves cut-side up on a tray and spoon the quinoa mixture into each one, pressing gently so it's packed but not overflowing. A sprinkle of crumbled goat cheese on top finishes each pepper beautifully before cooking.
If grilling, I place the peppers on soaked cedar planks over medium heat with the lid closed for 15 to 20 minutes. If using an oven, I bake them at 400°F for about 20 to 25 minutes until the peppers are tender and the filling is warmed through.
Once they're done, I let the peppers sit for a few minutes before serving. This little pause helps the filling settle and makes them much easier to lift off the tray without falling apart.