What must she do before using this spatula to assemble the completed burgers

Answer: Clean and sanitize it.

Explanation:

She must clean the spatula before using it to assemble the completed burgers, because it's not a good sanitary habit (it's against food safety) to use the same spatula to everything, in this context, they usually have one spatula for each stage of the preparation.

These actions are due to fulfil food safety in fast food restaurants, because the food can get bacterias or other micro organisms that are not good for human consumption. Even this can happen more frequently on fast food businesses.

Actually, fast food restaurants are constantly watched (they didn't before, years go, only usual restaurants were monitored), because food safety in those type of business can be infringed so easily, like cleaning a spatula with pants for example.