Cook the bacon: In a skillet over medium heat, cook the bacon until crisp. Transfer to a paper towel-lined plate and crumble when cool.
Reserve a teaspoon of bacon fat if you like added flavor in the dressing.
Toast the nuts: In the same pan (wipe if needed), lightly toast the pecans or walnuts over low heat for 2–3 minutes, stirring often. Remove from heat and let cool.
Make the dressing: In a small bowl, whisk together olive oil, lemon juice, Dijon, garlic, salt, and pepper. If using, whisk in up to 1 teaspoon bacon fat for extra richness.
Adjust seasoning to taste.
Prep the salad base: Place the spinach in a large bowl. Add red onion slices and cherry tomatoes if using.
Add the toppings: Gently fold in avocado, crumbled bacon, toasted nuts, and cheese. Nestle the egg quarters on top.
Dress and toss: Drizzle the dressing over the salad.
Toss gently to coat, being careful not to mash the avocado. Taste and add a pinch more salt or lemon if needed.
Serve: Divide into bowls and enjoy immediately for the best texture and flavor.