In a large nonstick skillet, heat the oil over medium heat. Add the sausage and cook, stirring and breaking it up into smaller pieces, until it begins to brown, 3 to 5 minutes. Add the onion and bell pepper and cook until the onion is golden, about 5 minutes. Add the garlic and cook until fragrant, 45 to 60 seconds. Remove from the heat to cool slightly.
Meanwhile, whisk together the eggs, milk, sage and parsley in a large bowl. Add the cubed bread, sausage-vegetable mixture and 1/4 cup of the cheese. Season with salt and pepper. Stir to combine. Cover and refrigerate for at least 30 minutes or overnight.
Preheat the oven to 350 degrees F. Brush a medium casserole dish with oil. Pour the chilled bread mixture into the prepared casserole dish. Sprinkle with the remaining 1/4 cup of cheese. Let sit at room temperature for 20 minutes.
Transfer the casserole to the oven and bake until puffed and golden brown, about 30 minutes. (If the top of the casserole starts to get too brown, cover it with aluminum foil.) Remove to a rack to cool slightly. Serve hot or warm.