In a large skillet over medium heat, add a thin drizzle of olive oil. Add bread slices and cook until golden brown and crisp, about 2 to 3 minutes per side. Transfer to a plate.
In a medium bowl, mash avocado with lemon juice. Season to taste with salt and freshly ground black pepper (I like to start with 1/4 teaspoon salt and 1/8 teaspoon pepper).
Evenly spread avocado mixture on the warm toast. Sprinkle with red chili flakes, if using, and serve immediately.
Notes
Toasting bread: I prefer to toast the bread in a skillet with olive oil for maximum flavor and crisp edges (butter works too). You can also toast bread in a toaster or bake it on a baking sheet in a 350-degree oven until golden.