Season the steak on both sides with salt and then coat with the peppercorns. Whisk the vinegar and maple syrup in a large bowl and add the shallot; set aside.
Heat a large skillet over medium-high heat and add 2 tablespoons vegetable oil. Add the steak and cook, undisturbed, until a crust forms, 4 to 5 minutes per side, plus about 1 minute on each of the long edges. Transfer to a cutting board to rest.
While the steak cooks, heat a second large skillet over high heat. Add 2 tablespoons vegetable oil and the mushrooms; cook, undisturbed, until starting to brown, about 3 minutes. Toss, season with salt and continue cooking until well browned and tender, about 5 more minutes.
Whisk the mustard into the shallot mixture, then whisk in the remaining 1 tablespoon vegetable oil. Add the mesclun, parsley and sautéed mushrooms and toss; season with salt.
Divide the salad among plates and top with the cheese. Slice the steak and add to the plates.