Cook bacon: In a skillet over medium heat, cook bacon until golden and crispy. Transfer to a paper towel-lined plate.
Boil eggs: Place eggs in a saucepan, cover with water, and bring to a boil. Reduce to low and simmer for 9–12 minutes. Transfer to ice water, then peel and slice lengthwise.
Grill steak: Preheat grill or grill pan to high heat. Season steak with salt and pepper. Grill to desired doneness, then rest a few minutes before slicing thin against the grain.
Make dressing: Whisk Dijon mustard, garlic, lemon juice, apple cider vinegar, and olive oil in a bowl until combined.
Assemble salad: Divide lettuce onto plates. Arrange eggs, bacon, tomatoes, cucumber, avocado, onion, and blue cheese over top. Place sliced steak on salad and drizzle with dressing.
Make sparkling chia fresca: combine all the ingredients. Stir and let sit for 5 minutes for the chia to bloom.
Serve Cobb salad with watermelon and sparkling chia fresca.