Heat a large cast iron skillet over medium-high heat with the olive oil.
When the skillet is very hot, but not yet smoking, add the thinly sliced ribeye to the skillet in a single layer. Season with the salt and pepper. Without touching the steak, sear the steak until golden brown for 2 minutes.
Flip the steak slices and continue to cook until the other side is golden brown, 1-2 more minutes. Transfer to a plate, set aside.
With the skillet still over medium-high heat, add the onions, bell peppers, garlic, coconut aminos, and fish sauce. Cook, stirring, until the veggies are slightly tender, about 3 minutes.
Add the mushrooms and continue to cook until all of the veggie are very tender and golden, 4-5 more minutes.
Add the steak back into the skillet with any of its juices and toss to combine. turn off the heat and lay the slices of cheese on top of the steak and veggie mixture and allow to just melt.
While the cheese is melting, warm your tortillas. You can do this 2 ways:1. For a Gas Stovetop: Fire up the burners on your stove to the lowest setting. One at a time, carefully place a tortilla directly on the burner and flip after about 45 seconds. As you finish cooking the tortillas, place them onto a towel and close the towel up to steam the tortillas and keep them warm.2. For an Electric Stovetop: Place the tortillas in a dry (no oil) stainless steel skillet over medium heat and cook them for about 30 seconds on each side.
Serve the philly cheesesteak mixture in the warm tortillas. Enjoy!!