Sear bacon in a heavy-bottom skillet wide enough to fit the whole strip over medium heat until crispy, remove from pan, pour all the rendered fat from the pan into a mixing bowl (but don’t wash the pan), whisk in lemon, mustard into the rendered fat and taste for salt.
Heat up the skillet with a little bacon fat left in it and crack 4 eggs into the remaining fat.
Assemble the salad by tossing the spinach in the warm bacon dressing. Pile into two deep bowls and top with the avocado, bacon strips and eggs divided between the bowls.