Preheat the broiler. Lay the polenta slices on a baking sheet and brush the tops with 1 tablespoon olive oil; season generously with salt and pepper. Broil until browned around the edges and crisp, 12 to 14 minutes.
Meanwhile, season the pork with salt and pepper. Heat a large nonstick skillet over medium-high heat and add 1 tablespoon olive oil. Add the pork and cook until browned, about 2 minutes per side; remove to a plate. Add the remaining 2 tablespoons olive oil, the mushrooms, sliced garlic, shallot and 1/2 teaspoon salt. Cook, stirring occasionally, until the mushrooms brown in spots, 6 to 8 minutes. Sprinkle with the flour and cook, stirring, about 30 seconds. Add the chicken broth and wine; bring to a simmer and cook until the sauce thickens, about 5 minutes.
Return the pork and any juices to the skillet. Cook over low heat until the pork is cooked through, 1 to 2 minutes. Stir in the orange juice and season with salt and pepper.
Combine the orange zest, parsley and minced garlic in a small bowl. Divide the polenta among plates. Top with the pork, mushroom sauce and parsley mixture.