how do you refactor c code that returns a listt?

The variable parts are the return type and the URI, so if you declare your method like this:

private async Task<List<T>> Get<T>(string requestUri)
{
    try
    {
        var client = ClientFactory.CreateClient("ServerAPI");
        return await client.GetFromJsonAsync<List<T>>(requestUri);
    }
    catch (AccessTokenNotAvailableException exception)
    {
        exception.Redirect();
    }
    return new List<T>();
}

Then you can call like this:

masterData.PatternData = await Get<MeetingPatternData>("MeetingTimeManager/GetPatterns");
masterData.PatternDataDays = await Get<MeetingPatternDays>("MeetingTimeManager/GetDays");

I’ve removed this part:

var retVal = new List<MeetingPatternData>();

Because your allocating a List unnecessarily.

You should probably also be returning an IEnumerable<T> rather than List<T> based on the philosophy of using the most restrictive type necessary to do the job, unless the caller needs to manipulate the returned data.