Refrigeration time: 30 minutes to 1 hour
Add oil and butter in a pan and place on medium-high heat. Add the minced garlic. Once it cooks, add the veggies, except the onions. Sautee till the veggies are half cooked and there is no water.
Add the spices and mix well for about a minute, then add the onions and fry for another minute.
Now add the mashed potatoes and incorporate everything well with a potato masher or flat ladle and mix for an additional minute.
Mix all-purpose flour + corn starch + cornmeal + salt and mix well. Add water little at a time until a slightly thick batter is formed. Mix well and stir the oil in the batter. (Oil adds shine to the crumb coat)
Pour oil in a pan and place on medium heat. (I used oil only enough to submerge half of the patty. You can use more oil and deep fry). Test if the oil is hot enough by adding bread crumbs. If it sizzles, that means the oil is ready for frying. Add the burger. After 2-3 minutes turn the burger over. Fry on both sides till the burger is golden brown. Transfer to a paper towel.
Cut a dinner roll into half. Place the burger on the lower bun/dinner roll. Spread some mayo on the upper side. Slide-in lettuce or leaves so that it doesn't make the burger soggy.
To freeze: Line a sheet or a plate with plastic wrap and place the coated patty on the sheet, keeping enough space so that they don't touch. Once it hardens in an hour or two, remove and place it in freezer bags or freezer boxes. When ready to use, thaw in the refrigerator (If you transfer the burger from the freezer to the refrigerator at night, it should be ready for frying by morning). Alternately you can place in the counter to thaw for 30 minutes to an hour or thaw in the microwave for few seconds.
You can also use frozen mixed veggies.