In a medium pan, saute onions and garlic in oil over medium heat until they begin to soften. Drain tomatoes, saving the juice, and add tomatoes to onions. If using whole canned tomatoes or fresh tomatoes, dice into chunks before adding to onions. Add the tuna, quinoa and water, mix to combine. Season with salt and pepper to taste. Cover and simmer for about 15 minutes, or until quinoa is cooked through.

On a grill, over an open gas stove flame or under a broiler, char the peppers until the skins are dark and blistered, turning often. Place in a ziploc bag, or a bowl and cover with plastic wrap to steam for 5-10 minutes, then peel off the skins. Make a small slit down the length of the pepper and carefully remove seeds and ribs. Stuff peppers with tuna filling, top with cheddar cheese and broil until the cheese is bubbly and melted.

In a small saucepan, combine tomato sauce and cumin over low heat. Thin out the sauce with reserved juice from the canned tomatoes (or water) until it reaches desired consistency. Season with salt and pepper, taste and adjust spices as needed. Ladle sauce onto bottom of serving dish, top with stuffed peppers and serve with sour cream.