Simple Marinated Slow Roasted Onions

These Marinated Slow Roasted Onions are not your ordinary side dish. Sometimes I get bored of the same baked potatoes, rice or broccoli, so this is the recipe that takes me out of the routine. Also, I love onions in every form possible, so these are the perfect addition to my dinner meal. I think caramelised onions are the best to put in sandwiches, burgers or on pizza. But what about pickled onions? They are simply delicious! The contrast between sweet and tangy is incredible.

So why not both caramelised and pickled? These roasted onions are so creamy and soft on the inside, super crisp and caramelised on the outside. Their distinctive tanginess is achieved with the addition of red wine vinegar in the marinade.

slow roasted onions

Ingredients used in Marinated Slow Roasted Onions

  • Red Wine Vinegar: This adds acidity and a tangy flavour.
  • Brown Sugar: Adds sweetness, starts the caramelisation process and balances the vinegar.
  • Fresh Thyme: Flavour and fresh, minty taste. Instead, you could use rosemary.
  • Mustard Seeds: This spice is a staple in any marinade. I recommend also adding some fennel seeds, coriander seeds and allspice.
  • Onions: The star vegetable in this side dish.
  • Butter: Adds lots of flavour and richness.

These roasted onions are marinated for at least 4 hours, but I strongly recommend to let them in the brine overnight. As a result, the flavours will meld together and the spices add incredible depth.

Slow Roasted Onions

This roasted onions recipe needs minimal prep work and attention. Just throw the marinade ingredients together, slice the onions horizontally and let them rest in the refrigerator for as long as you please (overnight is best). Then, butter them up and roast them in the oven for 1 hour, basting once or twice in the meantime. It’s that easy!

I like to keep the skin on for this recipe because it protects the outside from burning and it helps in keeping them together during the roasting process. They might pop over, though. If they do, gently press them into place with the back of a spoon.

You can eat them as a side dish with any piece of meat but I love using them in sandwiches as a special garnish.

5 from 3 votes

Simple Marinated Slow Roasted Whole Onions

Creamy, sweet and soft insides, crisp and caramelised outsides and a tangy sauce. These marinated slow roasted onions are the perfect side dish.
Servings 6
Prep Time 5 mins
Cook Time 1 hr
Resting Time 4 hrs
Total Time 5 hrs 5 mins



  • 240 ml water
  • 100 ml red wine vinegar
  • 3 tbsp brown sugar
  • 1 tsp fresh thyme rougly chopped
  • ½ tsp salt
  • ½ tsp black pepper freshly ground
  • 1 tsp mustard seeds
  • 4 large yellow onions


  • 50 g unsalted butter cubed
  • 1 tsp fresh thyme roughly chopped


  • Mix marinade ingredients in a baking pan that can hold 8 onion halves.
  • Rinse the onions, trim the ends and cut them in half, horizontally. Leave the skin on. Place them in the marinade with the cut side down. Marinate in the refrigerator for at least 4 hours, overnight if possible.
  • Preheat oven to 200°C. Turn the onions with the cut side up. Top each onion half with a slice of butter and some fresh thyme.
  • Roast for 1 hour or until golden brown. Baste 1-2 times during roasting. If they pop over, gently push them back with a spoon.
  • Remove from oven and peel the skin before serving. Top with fresh parsley or thyme.
Course: Side Dish
Keyword: healthy, onion, roasted, side dish, thyme
Slow Roasted Onions

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