Step 1: Mash the avocado
Cut the avocado in half, remove the pit, and scoop the flesh into a medium bowl. Mash with a fork until it reaches your preferred texture.
Step 2: Season the avocado
Stir in the lime juice, ½ teaspoon kosher salt, and crushed red pepper, if using. Mix until the seasonings are evenly distributed.
Step 3: Slice the tomato
Cut the tomato into 4 thick slices. Set aside the top and bottom pieces for another use.
Step 4: Toast the bread
Toast the artisan bread until golden and crisp enough to support the toppings. Place one slice on each of two serving plates.
Step 5: Add the avocado and shallots
Divide the mashed avocado evenly between the slices of toast. Spread it to the edges, then scatter the thinly sliced shallots over the avocado.
Step 6: Layer on the tomato
Place 2 tomato slices on each piece of toast. Sprinkle the remaining ¼ teaspoon kosher salt evenly over the tomatoes.
Step 7: Finish and serve
Drizzle the Tomato Avocado Toast with balsamic glaze. Add cracked black pepper and, if using, the sliced basil.
Serve immediately while the bread is crisp and the tomatoes are fresh.