Is it religiously acceptable for a single female colleague to repeatedly prepare food for the questioner's fiancé and his professor at work, ask for their opinion on it, and learn a new dish to bring for them, or is this considered overstepping the boundaries of interaction between men and women, especially since their work is in medical research and she is not the only woman in the office?

There is no harm in a woman preparing food for her colleagues at work, as long as there is no suspicion. Everyone must adhere to the Sharia (definition)-compliant guidelines in interactions and avoid anything that leads to temptation, such as free mixing and jesting. It is advisable to hasten to complete the marriage, due to the chastity it provides for both spouses, psychological stability, and the attainment of offspring.