Cut zucchini into noodles using a spiralizer. Place noodles in a colander set in the sink. Season lightly with salt. Let stand for 20 to 30 minutes.
Preheat oven to 375 degrees F (190 degrees C). Grease a baking sheet with cooking spray.
Spread onion slices on one side of the baking sheet. Spread radishes on the other side of the baking sheet. Drizzle 1 tablespoon olive oil on top.
Combine kosher salt and lemon zest in a small bowl; rub together with your fingers until fragrant. Sprinkle over onion and radishes.
Bake in the preheated oven until onion and radishes are tender and slightly browned, about 25 minutes.
Heat a large skillet over medium heat; grease with cooking spray. Cook and stir garlic until fragrant, 3 to 5 minutes. Drizzle in 1 tablespoon olive oil. Add onion and radishes; cook and stir until browned, about 3 minutes. Stir in dill weed.
Mix zucchini noodles into the skillet; cook until heated through, about 3 minutes. Season with salt and pepper. Remove from heat; pour lemon juice on top.