Heat a cast-iron skillet or nonstick pan over medium. Brush both sides of each bread slice with 1/2 tbs olive oil (reserve remaining oil) and, if using, lightly spread a thin film of butter on one side. Place slices in the skillet and toast 2–3 minutes per side, pressing gently, until golden and crisp. Transfer to a rack so bottoms don't steam and set aside.

Avocado Smash

Halve, pit and scoop the avocado flesh into a medium bowl. Add lemon juice, salt, pepper and red pepper flakes. Using a fork, smash to a spreadable but slightly chunky consistency. Taste and adjust seasoning. Keep covered until ready to assemble to prevent browning.

Toppings

Prepare tomatoes and mozzarella

Slice the heirloom tomatoes into 1/4-inch rounds and lightly salt them (this brings out sweetness). Tear or slice the fresh mozzarella into pieces about the size of the tomato slices so each bite will have cheese and tomato.

Assemble the toasts: Spread about 2 tbs of the smashed avocado evenly over each toasted bread slice (adjust to preference). Arrange tomato slices on top of the avocado in a single layer, overlapping slightly if needed. Tuck pieces of fresh mozzarella among the tomatoes so each toast has 2 oz of cheese.

Scatter torn basil leaves and the optional chopped oregano or chives over each assembled toast.

Garnish & Finish

Finish each toast with a light drizzle of the remaining 1 tbs minus 1/2 tbs olive oil (approximately 1 tsp) or the 1 tsp reserved for finishing, a drizzle of balsamic glaze (if using), a pinch of flaky sea salt and freshly ground black pepper. Add microgreens or baby arugula atop for brightness.

Serve immediately while the toast is crisp. If making ahead, keep toasted bread separate and assemble within 15 minutes of serving to avoid sogginess.