In a small bowl combine 3 Tbsp butter and 2 Tbsp basil; set aside. Sprinkle steaks with salt & pepper.
In a large skillet cook garlic in hot olive oil and remaining 1 Tbsp butter over med-low heat for 4-6 minutes or until garlic begins to brown, stirring occasionally. With a slotted spoon remove the garlic from the skillet. Increase heat to med-hi and add remaining 1 tbsp basil. Cook steaks in skillet, turning once. Allow 10-13 minutes for med-rare to med doneness. Transfer steaks to serving platter; top each with reserved butter & basil mixture.
Add mushrooms to skillet. Cook and stir until tender and brown, about 3 minutes. Spoon mushrooms over steaks.