Marinate the steak. Slice the steak as thinly as you can, making sure you cut against the grain. Add the steak to a large bowl. Add the cornstarch, egg white, vinegar, worchestershir, montreal steak spice and herbs de province. Mix very well till all the beef is coated. Set aside for at least 30 mins.
Just before you are ready to cook the veggies and meat, make the cheese sauce. Add the butter and flour to a small pot over medium-low heat. Stir till the butter is melted and the flour is all mixed in. Cook the butter and flour for two mins but be careful that it doesn't get too brown. Slowly add the milk bit by bit, stirring to combine. Then add the cream cheese, stir till its melted. Then add the grated cheese and stir until that is melted as well. The cheese sauce should be thick and very cheesy. Take it off the heat, put a lid on it and set aside. Try not to eat it all before you get the meat cooked :)
When you are ready to cook, heat a large skillet on medium heat. Add the olive oil, chopped onion, salt, garlic and jalapeno. Saute until the onions are translucent. Add the peppers and stir for a minute or two.
Pour veggies into a large bowl.
Drain the meat of any liquid and add it to the same skillet. Stir fry it until all the pieces are just cooked through. Do not overcook. Add the veggies back to the pan and mix well. Remove from the heat.
Turn your oven to 350 degrees. Add the portobello mushrooms to the pan and bake them for about 8 mins or so. Just so they start to get warmed through.
Pull them out of the oven and pile each one with the steak mixture. Top with the cheese and throw them back in the oven. Bake for about 10 mins or until the cheese starts to get bubbly. If you like, turn your oven on broil for a quick min to brown the cheese if it hasn't browned already. Be careful not to burn it.
Pull them out of the oven, top with chives if you wish and serve immediately.