Pre-heat oven to 350 degrees.
Brush bread slices lightly with about 2 tsp olive oil and place on a foil or parchment paper-lined baking sheet.
Bake at 350 degrees for 16-18 minutes, until just crispy.
While crostini are in oven, mix together butter, 4 cloves minced garlic, and 1 ½ tsp chopped basil in a small bowl.
When crostini are out of oven, lightly brush butter mixture onto each piece of bread. Set aside.
Pour remaining 2 Tbsp olive oil into large skillet or dutch oven over medium heat. Add shallot and cook for about 2 minutes. Add remaining 3 cloves minced garlic and cook for 2 more minutes.
Add the mussels to the pan, along with the Sonoma Brut and lemon juice.
Bring to a boil, cover pan, and let cook for 8-10 minutes, until mussels have opened. Discard any mussels that have not opened. Place cooked mussels in a large bowl.
When the mussels are cool enough to handle, remove meat from the shells.
Top each piece of bread with diced avocado and three mussels.
Evenly distribute lemon zest and remaining Tbsp chopped basil on top of crostini and squeeze grilled lemon over the top.