In a small food processor or blender combine 1 cup fresh basil leaves, 1 peeled garlic clove, 2 tbsp pine nuts, and 2 tbsp grated Parmesan. Pulse to chop. With the motor running, drizzle in 3 tbsp extra-virgin olive oil until a coarse paste forms. Add 1 tsp salt, 1 tsp black pepper and 1 tsp lemon juice (optional). Pulse once or twice to incorporate. Taste and adjust seasoning. If you prefer a thinner pesto, add a little more olive oil, 1 tsp at a time.

Balsamic glaze

Reduce balsamic

Place 1 cup balsamic vinegar and 1 tbsp sugar (or honey) in a small saucepan over medium-low heat. Stir until sugar dissolves and bring to a gentle simmer. Reduce heat to maintain a low simmer and cook, stirring occasionally, until the liquid is reduced by about half and coats the back of a spoon, about 12–18 minutes. Do not boil vigorously. Remove from heat and let cool; it will thicken further as it cools. Transfer to a small container and set aside.

Focaccia & assembly

Prepare focaccia

Preheat oven to 425°F (220°C) or set a skillet over medium-high heat. If slices are thick, halve them horizontally. Brush both sides lightly with 1 tbsp extra-virgin olive oil. Place focaccia slices on a baking sheet cut-side up (or in the hot skillet) and toast in the oven 5–7 minutes until golden and slightly crisp, or 2–3 minutes per side in the skillet. Remove and let cool slightly.

Slice tomato and mozzarella

While focaccia toasts, slice the tomato into 1/4-inch (6 mm) rounds and slice or tear the 8 oz fresh mozzarella into pieces roughly the same size as the tomato slices. Pat mozzarella dry with paper towel to remove excess moisture.

Assemble toasts

Spread about 1 tbsp basil pesto evenly over each toasted focaccia slice. Top pesto with a small handful (about 1/4 cup) baby spinach on each slice. Arrange tomato slices over the spinach (2–3 slices per piece depending on size), then place mozzarella slices on top of the tomato. Tuck 3 fresh basil leaves gently onto each assembled toast for extra aroma.

Season & finish

Lightly season each assembled toast with salt and freshly ground black pepper to taste. Drizzle balsamic glaze over the toasts in a thin line or zigzag (about 1–2 tsp glaze per slice). If desired, finish with a small drizzle of extra-virgin olive oil.

Serve

Serve the Caprese focaccia immediately while the bread is still slightly warm, or let cool to room temperature. Leftovers (sans pesto on raw produce) keep best stored separately: refrigerate assembled toasts up to 1 day and balsamic glaze up to 2 weeks refrigerated.