Make sure to have all your ingredients ready to go before starting. It is also important to read through the instructions carefully before beginning. Instructions for prepping the dish in advance are at the bottom.
Preheat oven to 375 degrees.
In a large saucepan, heat ¼ cup of olive oil over medium heat. Add the onions to the oil and cook, stirring occasionally for 2-3 minutes.
To the onions and olive oil add in your mushrooms and garlic. Allow them to cook, stirring occasionally for about 4-5 minutes until the mushrooms have softened.
Add in the 2 tablespoons of olive oil and sprinkle your flour over the top. Continue to mix, stirring constantly for about 3 minutes.
Pour in your stock and mix well.
Your remaining ingredients are next. Remember to add only 1 cup of gruyere cheese. Mix them well and taste. Taste and djust salt and pepper.
Pour the casserole mix into your baking dish. Top with the remaining ½ cup grated gruyere.
Place in oven and bake for 20-30 minutes. The dish is cooked all the way through, it just needs to be heated together.
Once the cheese is melted and the dish is bubbly, it is ready to go!!
Now, if you want to prep this dish in advance, it is very simple. Make the dish as above. When you finish assembling the dish, pour it into your baking dish. At this point, wrap with saran wrap or foil and place in the fridge. The dish can be made up to 2 days in advance. On the day of baking, preheat the oven. Add the casserole and bake until the cheese is melted and the dish is bubbly all the way through.