Pickling spice can be found in all major grocery stores. While it seems like kind of a speciality ingredient, it boasts the perfect blend of complex, savory flavors that round out this otherwise simple sauce. (The
blend I use includes cinnamon, allspice, mustard seed, coriander, bay, ginger, chilies, clove, black pepper, mace and cardamom.) You can grind the spices by hand using a mortar and pestle or molcajete, but I've found an electric spice grinder works best, particularly on the barkier bits. (Wondering what to do with the rest of your pickling spices? I love adding a tablespoon of whole spices to a solution of sugar, salt and water to brine pork chops or chicken before grilling or roasting!)
To toast walnuts: spread walnuts in a single layer on rimmed baking sheet and bake at 350F until toasty and fragrant, about 5-7 mins, keeping an eye on them. Alternatively, toast them in a single layer in a dry saute pan over med to med-high heat, tossing frequently.