Preheat oven to 425°.
Toast walnuts for 7 minutes. Let cool. Spread oats on a rimmed baking sheet and toast until lightly golden, about 12 minutes until oats are deep golden brown. Let cool. Whisk flour, salt, baking soda, cinnamon, and nutmeg in a medium bowl; stir in oats and walnuts.
Using an electric mixer on medium speed, beat butter, brown sugar, and granulated sugar in a large bowl, scraping down sides of bowl, until light and fluffy, 3–4 minutes. Add egg, treacle, and vanilla and beat until incorporated, about 1 minute. Reduce speed to low; add dry ingredients and raisin marmalata and continue to beat, scraping down sides and bottom of bowl, until just combined. Let sit at room temperature at least 1 hour to hydrate oatmeal.
Preheat oven to 350°.
Using a 2-oz. ice cream scoop, portion cookies and place on a parchment-lined rimmed baking sheet, spacing 3" apart (or measure out mounded scoops with a ¼-cup measuring cup). Do not flatten; cookies will spread as they bake. Bake cookies until edges are golden brown and firm but centres are soft, 15–17 minutes. Let sit on baking sheet 10 minutes, then transfer to wire racks; let cool. Store in an airtight container.
- Spice: spices can be increased or decreased to taste. For a more pronounced taste of the ras al-hanout, replace nutmeg with ⅛-¼ teaspoon (per taste) La Boîte Tangier #23.
- Texture: for chewier cookies, reduce baking time by 2 minutes. For crispier, flatten them a bit before baking.
- Do Ahead: cookie dough can be made 3 days ahead; cover and chill. Let dough come to room temperature before baking.
- 2 cups old-fashioned oats
- 1½ cup chopped walnuts
- 1 cup all-purpose flour
- 1 teaspoon sea salt
- ½ teaspoon baking soda
- ½ teaspoon ground cinnamon
- ⅛ teaspoon freshly ground nutmeg*
- ¾ cup (1½ sticks) unsalted butter, room temperature
- ¾ cup (packed) light brown sugar
- ¼ cup granulated sugar
- 1 large egg, room temperature
- 2 tablespoons treacle
- 1 teaspoons vanilla paste
- 12-16 oz spiced raisin marmalata with ras al hanout and smoked cinnamon