There isn't a common noun that comes to mind that means this, but several adjectives.
Periodically or gradually probably expresses what you mean the most succinctly:
Rather than putting the whole mixture of spices in at once while cooking something, periodically add part of the mixture while it cooks.
Rounds or steps refer to a process, not a thing.
The first thing that comes to my mind with the word round is a game of some sort. A situation where there are multiple people (or other entities) and everyone/each thing is taking/receiving a "turn". There is an implication that everyone gets a turn, but no implication on the time that may take. You only have a single thing (the dish) receiving a single action (adding spices) that is desired on a defined time frame so the word round is not suitable.
Steps means distinct actions to take to accomplish something. It does not by default suggest a single repeated action over time, but rather various different actions that must be completed in sequence. So in my opinion this word is not entirely suitable either.