Position a rack in the center of the oven and preheat to 325º F. Line a rimmed baking sheet with parchment paper. Have a second rimmed baking sheet and piece of parchment ready.
Combine the oats, nuts, sugar, orange, cinnamon, and nutmeg together in a large bowl. In a small saucepan, melt together the butter, maple syrup and salt, stirring to dissolve the salt. Pour over the oat mixture, tossing to combine thoroughly.
Dump the mixture onto the lined baking sheet, and spread it into a thin sheet no thicker than an almond. Ideally, the edges will be slightly thicker than the center as they will bake faster. Lay the second piece of parchment over the granola, and place the second sheet pan on top, right-side up, making a little granola sandwich.
Slide the whole thing into the oven and bake for 20 minutes. Rotate and continue baking another 10 to 20 minutes, peeking under the parchment to make sure the edges aren’t burning, until the granola is a rich golden brown. It will still be soft, but will crisp up as it cools.
Let the granola cool completely in the sheet pan sandwich, then break up and store in an airtight container for up to a month (or possibly more). Enjoy over fruit and yogurt for breakfast, alone for a crunchy snack, or over ice cream for a decadent dessert.