Preheat your grill to medium-high heat, about 400°F.
Cut the zucchini in half lengthwise. Using a spoon, carefully scoop out the center of each zucchini half, leaving about a ¼-inch border around the sides and bottom so the zucchini is sturdy enough to hold the filling.
Lightly brush both sides of the zucchini with olive oil and season with salt and pepper.
Place the zucchini cut-side down on the grill and cook for 3–4 minutes without moving them to develop those beautiful grill marks. Flip and grill for another 2–3 minutes.
Remove the zucchini from the grill. It should be starting to soften but still firm enough to hold its shape.
Meanwhile, whisk together eggs and milk.
Stir in cooked sausage, 3/4 cup cheddar cheese, green onion, garlic powder and black pepper.
Remove zucchini from the oven and blot away any moisture that has collected inside.
Divide egg mixture among the zucchini boats. Sprinkle with the remaining 1/4 cup cheese.
Bake for 15–20 minutes, or until the egg filling is completely set.
Rest for 5 minutes before serving. Garnish with additional chives or green onion if desired.
No grill? No problem! Preheat the oven to 400°F. Cut the zucchini in half lengthwise and scoop out the centers, leaving about a ¼-inch border. Lightly brush the zucchini with olive oil and season with salt and pepper. Place the zucchini cut-side up on a baking sheet or in a baking dish and par-bake for 10–12 minutes, just until it begins to soften but still holds its shape. Remove from the oven, add the sausage, egg and cheese filling, then return to the oven and continue baking as directed in the recipe.