Preheat oven to 300°F and line two baking sheets with parchment paper.
In a small sauce pan, melt together butter and honey.
Mix together oats, nuts, and salt in a large bowl. Drizzle butter mixture on and stir well to coat.
Spread onto baking sheets and bake for about 30 minutes, rotating baking sheets halfway through.
Remove from oven and immediately scatter blueberries and lemon zest, if using, over the top of the granola. Stir quickly and then press granola down, to help form large chunks.
Let cool completely, and store in airtight containers, preferably glass.