Put the washed spinach in a saucepan with a lid and cook until wilted, 2-4 minutes, or put the washed spinach in a microwave-safe covered dish and cook on high until wilted, 1-3 minutes, taking care not to overcook.
Put the mayonnaise, mustard and 2 tbsp lemon juice in a small bowl, and blend well; aim for a definite lemon flavour, so add more juice if it isn't noticeable.
Cut the croissants in half lengthwise and place them cut-side up on plates.
Drizzle each with 1/8th of the sauce, (put tomato slices on now if you want) divide spinach into four portions and place on croissant halves, and drizzle the rest of the sauce on the spinach.
Poach the eggs: I am hopeless at poaching them in a saucepan, so I do mine one-by-one in the microwave in a small lidded ceramic bowl; you could use ramekins or another small dish.
Grease the inside of the bowl with a bit of the butter, add 1 tbsp cold water and one egg; pierce the yolk lightly with a fork so it doesn't explode.
Cook on medium for 30 seconds, check for doneness (set white, still soft yolk) and if necessary cook in 10-15 second increments until the white is fully set, but keep the yolk pretty soft.
Put one egg on each croissant, and serve.
p.s. If you are more ambitious, make real hollandaise sauce, such as #49031, instead my cheat!
p.p.s. When I am REALLY lazy, I just use lightly browned toast instead of the croissant, but it's not nearly as nice.