I have some Florence fennel in my garden that tends to grow very fast in height and to make flowers, but the base stays relatively thin.
What should I do to encourage the growth of a "fat" bulb in order to have a better harvest? Does it help to cut it from time to time?
Florence fennel is very fussy and prone to bolting when temperatures are variable, so it could be bolting if you're sowing/planting it new each year. In this case, I suggest trying to protect it from temperature extremes. If you've been growing fennel in the same position year upon year, it could be that the soil needs enriching. Also, it's a biennial, so it's unlikely to produce sufficiently inflated stems after its first year.
