Cook potatoes in boiling water or microwave. Roughly smash with a fork potato into large bite sized pieces. Stir in 1 tbsp oil and thyme. Set aside
Thinly slice onion and fennel bulb. Crush garlic.
In oven proof skillet on medium heat, add 1 tbsp oil and 1 tbsp butter. Add onions and 1/2 tsp salt. Cook until onions are caramelized being sure to stir often so that the onions do not burn. Stir in ground pepper and add to the potatoes.
In same skillet, heat remaining 1 tbsp oil and add fennel, garlic, and 1/2 tsp salt. Cook until brown. Set aside with the potato and onion mixture.
Again in the same skillet, cook sausages over medium heat. Halfway through cooking, remove sausages and slice into 1/2 cm medallions. Return to skillet and pan fry until slices are brown and slightly crispy on the edges.
Add potato, onion, fennel mixture to the skillet with the sausages. Lightly stir being careful not to break down the potatoes too much and put into preheated oven.
Gently stir mixture every 15 minutes, until potatoes are nicely browned. Approximately 50-55 minutes depending on the size of your pan.
Stir in fennel fronds. Serve with sour cream or ketchup.