Add water to the bowl of a stand mixer. If using active (not instant) yeast, add yeast and sugar and let sit for 10-15 minutes until bubbly. If using instant yeast, add yeast and sugar and continue to the next step.
Add the flour, fennel seeds, salt, onion, and garlic. Mix on low to combine then on medium for 5-10 minutes until incorporated and smooth, not very tacky.
Transfer to a lightly-greased bowl and cover with greased plastic wrap. Let sit for 10-14 hours overnight.
In the morning, turn out onto a lightly flowered surface. Knead and form into a boule on parchment paper. (Optional: Use a banneton and when done rising invert onto the parchment paper.) Let rest/rise for 30-40 minutes while the oven heats.
Heat oven to 450°F / 230°C
Put your Dutch Oven and lid into the oven to heat with the oven. Wait until it's been at temperature for 10-15 minutes before using.
Make a couple of slashes in the top of the bread with a sharp knife or lame.
Remove the Dutch Oven from the oven, the lid from the oven, and transfer bread and parchment into the Dutch Oven.
Replace the lid and put the Dutch Oven back into the heated oven.
Bake for 30 minutes.
Remove the lid and bake another 15-20 minutes until fully baked and internal temperature reaches 205°F / 96°C
Transfer to a cooling rack and let cool an hour before slicing.