Place half the sliced onions into the bottom of a slow cooker. Season the roast on all sides with 1 teaspoon of sea salt and 1 teaspoon of black pepper then lay the roast on top of the onions. Place the remaining onions and garlic on top of the roast.
In a large bowl, mix the red wine, diced tomatoes, beef stock, Worcestershire sauce, rosemary, oregano, basil, thyme, 1 teaspoon sea salt and 1 teaspoon black pepper. Pour the wine mixture over the roast. Cover the slow cooker. Set the cooker to low then cook until tender, 8 to 10 hours.
If desired, thicken the sauce at the end of cooking by whisking tomato paste with juices until thoroughly combined. Set the cooker on high heat and cook an additional 10 minutes.
Use a full-bodied red wine. Using non-alcoholic wine is perfectly fine but I suggest pouring 2 tablespoons of cider or balsamic vinegar into measuring cup then adding wine to make 2 cups. Non-alcoholic wine tends to be on the sweet side; the vinegar helps to balance the flavour.