Bring a large pot of water to a boil, add in a tablespoon of salt to the water, and cook the pasta until al dente. Strain out the cooked pasta and reserve.
Heat large skillet or Dutch oven on medium-high heat, and drizzle in the 2 tablespoon of olive oil. Place the onions, garlic and mushrooms in the pan with the 1 teaspoon salt and the oregano. Cook until the mushrooms have reduced and most of the liquids have evaporated. Scoop out of the pan and place in bowl.
In the same skillet, drizzle in a tablespoon of olive oil and place the cubed SPAM in the pan. Fry until the Spam is nice and crispy on all side. Remove from the skillet and add to the bowl with the veggies.
Reduce the heat to medium. Place the butter in the skillet and use it to deglaze the pan and scrape up in stuck on bits in the pan. Whisk in the flour into butter to get a smooth mixture (roux). While whisking continuously, slowly pour in the milk and bring to a slow simmer.
Add the cubed cream cheese into the skillet and whisk until it melts in the sauce. Once completely incorporated, turn off the heat.
Add the peas, cooked veggies, fried spam, and cooked pasta, into the sauce. Stir everything together to coat.
Preheat the oven to 400F. Place the pasta in a 2-quart casserole dish. Mix together the parmesan, panko crumbs and chopped parsley in a dish. Sprinkle the mixture over the pasta and place the casserole in the preheated oven for 15 minutes.
Take the dish out of the oven and rest for 5-10 minutes before serving. Enjoy!
Notes
Quick weeknight meal: After adding back the veggies, fried spam and pasta to the sauce, the Tetrazzini can be served. Simply omit the parmesan and panko crumbs from the recipe.