3 tablespoons (14 grams) olive oil, divided
1 pound (450 grams) lean ground beef
2 tablespoons (30 ml) butter, divided
2 onions, sliced
1 teaspoon (5 ml) brown sugar
2 teaspoons (10 ml) salt, divided
2 teaspoons (10 ml) black pepper
1 cup (340 grams) mushrooms, quartered
1 18-ounce (510 grams) can of beef gravy
5 russet potatoes, thinly sliced to about 1/4" (1/2" cm)
1 cup (450 grams) frozen corn
1 10-ounce (283 grams) package frozen spinach, thawed and squeezed to remove excess moisture
1 cup (340 grams) Swiss cheese, shredded
Preheat oven to 350 degrees Fahrenheit (180 degrees Celsius).
Heat one tablespoon (15 ml) of olive oil in a skillet set over medium heat. Add ground beef, season with 1 teaspoon (5 ml) of salt and 1 teaspoon (5 ml) black pepper. Cook, stirring occasionally, until ground beef is no longer pink. Drain of excess fat and set aside.
Heat one tablespoon (15 ml) of olive oil in a skillet along with 1 tablespoon (15 ml) of butter over low heat. Add the onions, toss them in the fat, and season with 1/2 teaspoon (2.5 ml) of salt and 1/2 teaspoon (2.5 ml) of black pepper. Sprinkle brown sugar over top. Cook, stirring regularly, for about 20 minutes, until the onions soften and turn golden brown. Set aside.
Heat one tablespoon (15 ml) of olive oil and one tablespoon (15 ml) of butter in a separate skillet set over medium heat. Add mushrooms, toss in fat, and cook for about 10 minutes, until the mushrooms begin to brown. Season with 1/2 teaspoon (2.5 ml) of salt and 1/2 teaspoon (2.5 ml) black pepper. Set aside.
Spread about 1/2 cup (170 grams) of beef gravy into the bottom of a 9" x 13" casserole dish.
Layer half of the potato slices onto the gravy, along the bottom of the casserole dish. On top of these, add the ground beef, then the onions, then the mushrooms, the frozen corn, and the frozen spinach. Add the rest of the potatoes on top (in as much of a single layer as possible), and pour the rest of the gravy over top of the entire dish. Sprinkle Swiss cheese over top.
Place the casserole dish into the oven and bake, uncovered, for 1 hour, until the Swiss cheese is melted and the dish is hot and bubbling throughout.