I may have an opportunity to be in Europe in Mid December and looking for options for hiking tours. I am looking for recommendations on guided or self guided options. As I am doing my own searching I am not finding any available in December. Suggestions on tour companies? Preferrably ones with small groups.
Go Cross Country Skiing in Norway! You could also try snow shoes! I'm not joking, you might find some guided winter sports. I hear they do rent treking poles. Or you might tour all the Christmas Markets, dress warm! Europe had some cold and snowy weather last winter.
I would try something in Spain.
You might want to explore cross country skiing in the Bavarian Alps. Miles and miles of groomed trails of varying degrees of difficulty. You may also find snow shoe rentals available.
But I agree that for hiking in snow free areas, you would want to look in Spain south of the Pyranees, Portugal and mid to southern Italy.
I have taken two 100-mile-long hiking trips in Ireland. I suspect it would be hard to hike there in December, but if you want to inquire, a good group is Ireland Walk Hike Bike: https://irelandwalkhikebike.com/
I learned from a Belgian couple that there's a European Hiking Federation: https://www.era-ewv-ferp.org/
If winter hiking in snow and cold would appeal to you, trips in the Tatra Mountains in Poland and Slovakia would be good.
If hiking in snow is not what you're after then consider hiking in the mountains of Mallorca.