Cook the pasta according to the package
In a large skillet, heat on high. Add two table spoons of vegetable oil. Sauté the tri-tip for 3 to 5 minutes until both sides are brown. Set aside.
On medium high heat, add the garlic, tomatoes and mushrooms and cook, until there is some color on the mushrooms, about 2-3 minutes. Sprinkle on flour and stir to incorporate. Slowly stream in milk while whisking. Bring to a simmer and stir, until the sauce thickens slightly.
When the sauce is simmering, the pasta should be done cooking. Drain the pasta. Add the pasta to the sauce and add the fresh spinach, tossing, to wilt. Season with salt and pepper and enjoy!