Cook your eggs in a pan to your preference. To get a yoke-y sunny side up egg like the one pictured in this post, heat the oil in a medium skillet over low heat until the oil is shimmering just slightly. This should take about 5 minutes. About 3 minute in, toast your toast. (Or try to time it so that your toast is still warm by the time your eggs are ready. Every toaster is different.)
Once the oil is shimmering, carefully crack your egg into the pan; repeat with the other egg if you're using two. Cover with a tight lid and simmer, uninterrupted, until the whites are completely set. This should take 2 to 2.5 minutes. While the eggs cook, place the tomato-sauce slathered sardines onto one slice of toast, then top it with leafy greens and tomatoes. Slather the other slice of toast with mayo if you wish.
When the eggs are done, carefully transfer them to your sandwich, placing them over the tomatoes. Season with salt and pepper. If desired, place cheese over eggs with a drizzle of hot sauce.