Meanwhile, place 3 TBSP butter and minced garlic in a small, microwave-safe bowl. Microwave on high until butter melts, about 30 seconds. Place ciabatta halves cut-side up on a baking sheet and drizzle with butter mixture. Toast in oven until crisp and golden, about 5 minutes. Toss together apple, celery, arugula, sunflower seeds, 1 TBSP olive oil, and a squeeze or two of lemon in a medium bowl. Season with salt and pepper.