Prepare the bacon in a cold large skillet placed over medium heat, arranging 8 slices in a single layer without overlapping; cook and flip occasionally until fat is rendered and bacon reaches desired crispiness, about 10–14 minutes, then transfer to a plate lined with paper towels to drain.

Split 4 bagels and lightly butter the cut sides if desired, then toast face-down in the same skillet used for bacon for 1–2 minutes until golden, or toast in a toaster; sprinkle tops with everything bagel seasoning if using.

In a mixing bowl whisk together 6 large eggs with 2 tablespoons whole milk (or cream), 1 tablespoon water, a pinch of salt and a few cracks of black pepper until uniform and slightly frothy.

Heat a small nonstick skillet over low heat and add 2 tablespoons unsalted butter; once melted, pour in the egg mixture and let sit undisturbed for 10–15 seconds until edges set, then gently push edges into the center with a silicone spatula to form large curds; continue slowly stirring until eggs are custardy but slightly underdone, 2–4 minutes, and remove from heat.

If using cheese, place 4 slices on the bottom halves of the toasted bagels; divide the warm scrambled eggs among the bagel bottoms, top with two slices of drained bacon per sandwich, add mayonnaise or extra butter on the top halves if desired, then close and press lightly.

Wrap assembled sandwiches loosely in aluminum foil or parchment to keep warm for up to 30 minutes, or slice and serve immediately garnished with chopped chives and a sprinkle of seasoning if desired.