Make the goat cheese spread: In a small bowl, stir together the goat cheese, chives, olive oil, garlic, and onion powder until smooth and well-combined. Set aside while you prepare the eggs.
In a medium bowl, whisk together the eggs and salt.
Heat the olive oil in a large nonstick skillet over medium heat. Add the spinach and a pinch of salt and cook, tossing, until the spinach is just wilted. Arrange the spinach evenly at the bottom of the pan and reduce the heat to medium-low. Pour in the eggs and cook without stirring until the edges begin to set, about 30 seconds. Gently run a spatula around the edges of the omelet and tilt the pan so that uncooked egg from the top of the omelet runs under and around the edges. Repeat until the omelet is nearly cooked through, then fold the omelet in thirds to create a long rectangular shape. Cook for 30 more seconds, or until the eggs are set. Remove from the pan and slice crosswise into four rectangular pieces.
Assemble the sandwiches. Spread the goat cheese spread on each half of an English muffin. Place one of the omelet slices on the bottom half, then top with a tomato slice, red onion, avocado slices, and microgreens, if desired. Repeat to assemble the remaining sandwiches. Serve with hot sauce.
To make the sandwiches ahead: Fully assemble the sandwiches, omitting the avocado and microgreens. Wrap in foil and freeze for up to 3 months. To reheat a frozen sandwich, remove the foil and wrap in a clean kitchen towel. Microwave for 2 minutes, then let stand for 1 minute. If desired, add avocado and microgreens. Serve with hot sauce.