Put the sun-dried tomatoes on a small plate and microwave in 30-second increments, checking the tomatoes for texture in between, until they begin to harden but are still red and slightly pliable, 1 to 2 minutes (they may begin to brown and taste bitter if cooked too long). Let them sit until cool and completely hardened, about 2 minutes.
Break up the tomatoes into small pieces. Transfer them to a spice grinder and process to a powder (discard any pieces that won't grind). Add the Parmesan, oregano, garlic salt and 1/4 teaspoon kosher salt. Process until finely ground.
Whisk together the sour cream, mayonnaise and 6 tablespoons of the spice blend in a medium bowl until smooth. Season with kosher salt. Transfer to a serving bowl, sprinkle with additional pizza spice blend and serve with bread sticks, carrots, celery and/or string cheese.
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