Preheat oven to 350°F (175°C).

Cook spaghetti until al dente according to package directions.

Drain the pasta and set aside.

Heat vegetable oil in a large pan over medium heat.

Add chopped red bell pepper, sliced mushrooms, and cubed seitan to the pan.

Sauté the vegetables and seitan until the vegetables soften and the seitan begins to brown, about 8-10 minutes.

Remove the pan from heat.

In a small saucepan, bring vegetable broth to a boil.

Reduce heat to a simmer.

Slowly whisk in flour until smooth.

Continue whisking until the mixture thickens slightly.

Add vegan cream cheese to the sauce and stir until completely melted and the sauce is creamy.

Remove the saucepan from heat.

Grease a 9x9 inch baking dish with oil or margarine.

In a large bowl, combine the cooked spaghetti, sautéed vegetables and seitan, and the cream cheese sauce.

Add 2 teaspoons of vegan parmesan cheese (Parma) to the mixture and stir to combine.

Pour the mixture into the prepared baking dish.

Top with grated vegan mozzarella cheese.

Cover the baking dish with aluminum foil.

Bake in the preheated oven for 25 minutes.

Remove the foil and sprinkle the remaining vegan parmesan cheese over the top.

Let the tetrazzini cool for 5-10 minutes before cutting into squares and serving.