Brush the steaks on both sides with oil and season generously with salt and pepper. Allow the steaks to stand at room temperature for 15 to 30 minutes before grilling.
Prepare the grill for direct cooking over high heat (450° to 550°F) and medium heat (350° to 450°F) and preheat a perforated grill pan over medium heat.
In a skillet over medium-low heat, cook the bacon and onion until the bacon is crisp, 6 to 8 minutes, stirring occasionally. Remove from the heat.
Combine the vinaigrette ingredients, including ¼ cup oil, ½ teaspoon salt, and ¼ teaspoon pepper; whisk until smooth. Put the mushrooms in a medium bowl and add ¼ cup of the vinaigrette. Mix well. Reserve the remaining vinaigrette.
Brush the cooking grates clean. Grill the steaks over direct high heat, with the lid closed, until cooked to your desired doneness, 8 to 10 minutes for medium rare, turning once or twice. Remove from the grill and let rest for 3 to 5 minutes.
Spread the mushrooms in a single layer on the grill pan and cook over direct medium heat, with the lid closed, until golden brown and tender, 6 to 8 minutes, turning once or twice. Don’t move the mushrooms until the bottom sides are nicely browned. Transfer the mushrooms to the skillet with the bacon and onions and warm over medium heat for about 1 minute, stirring often. Add the chives and mix well. Serve the steaks warm with the mushroom mixture spooned on top. If desired, whisk the reserved vinaigrette and drizzle some over each steak.