Place portobello mushrooms in a shallow baking dish; set aside. In a small mixing bowl, whisk together balsamic vinegar, olive oil, cumin, black pepper, paprika, garlic and steak sauce (optional). Taste and adjust seasonings as needed. With a brush, baste the mushrooms on all sides. Marinate on each side for 5 minutes.
While mushrooms marinate, slice the potatoes in 4 or 5 vertical pieces; then cut each of piece into sticks. Place in a large bowl and cover with cold water. Allow them to soak 2 to 3 hours. (You can also stick them in the fridge and let them soak overnight.) When you're ready to make the fries, drain the water and lay the potatoes on a baking sheet lined with paper towels. Blot to dry.
Preheat oven to 200 degrees.
Place potatoes in a 4 to 5-quart heavy, deep frying pan or Dutch oven with enough oil to barely cover the potatoes. Turn burner to high and bring oil to a light boil. Cook potatoes for 15 minutes. You can use tongs or a thin spatula to gently scrape loose any potatoes that have stuck to the pan; try not to stir too much as they will break apart. Continue to cook, stirring only if absolutely necessary, for another 5 to 10 minutes or until potatoes are crisp and golden. Using a large slotted spoon, scoop potatoes from pan and spread on a paper towel-lined tray to drain. Toss immediately with salt.
Keep fries warm in the oven while you prepare your portobellos.
In a large skillet over medium heat, cook portobellos on each side for 2-3 minutes, or until caramelized and deep golden brown. Brush on any remaining marinade while cooking to infuse more flavor. Remove from pan. Deglaze pan with wine; and add onions. Sauté until most of the liquid is evaporated and onions are golden brown. Spoon onions over portobellos and serve with fries.