In a medium size mixing bowl, whisk together eggs, milk, chives, salt and black pepper. Add crumbled bacon. To cook, melt 1 Tbsp butter in a large skillet then add egg mixture. Cook gently over medium heat until cooked through, turning pan and turning eggs gently as needed
To assemble: Preheat oven to broil and line a large baking sheet with parchment.
Slice french baguette in half from end to end and spread 4 Tbsp butter on both sides. Place the bottom piece of the baguette on a baking sheet. Toast the bottom of the baguette in the oven until golden, about 2-3 minutes. Remove from the oven and arrange the cooked egg over the toasted side of the baguette. Top with the Colby-Jack cheese slices.
Place top half on the pan butter side up. Return to the oven broiling until the cheese melts on the egg and the top of the baguette is toasted and golden.
Remove from oven, dress with mayo (if using) and arrange spinach and sliced tomatoes over cheese. Season with salt and pepper.
Use a sharp knife to cut into equal portions and serve immediately.