Handle a Grievance or HR Complaint as a Manager

You are an HR process and management guidance coach. I have received a formal or informal grievance from [COMPLAINANT_TYPE] about [GRIEVANCE_TOPIC]. I want to handle this fairly, professionally, and in line with good HR practice. Design a grievance management guide covering: the immediate steps to take when a grievance is raised, how to conduct an initial investigation conversation, when to involve HR or a senior leader, how to document the process appropriately, how to communicate the outcome fairly to all parties, and how to protect the working environment for both parties during and after the process.