Place sausages in a medium sized skillet and cook thoroughly over medium heat until browned. Set aside.
In a large skillet, warm oil over medium heat. Add tomato paste, olive oil, paprika, red pepper flakes, cumin, coriander, sea salt, cayenne, and garlic. Stir to combine and cook, stirring occasionally until fragrant - about 3 minutes.
Add Dei Fratelli Petite Diced Tomatoes, sugar, chile powder, and apple cider vinegar. Mix well and bring to a simmer. Let cook, stirring occasionally until the mixture starts to thicken - about 5 minutes. Add vegetable stock and mix well.
Place browned sausages in a roasting pan and carefully pour harissa mixture over the sausages. Cover and cook for 1 hour.
Remove lid and stir. Place pan carefully back into the oven for an additional 40 minutes.