Ginger, garlic, and onion: add ginger and garlic cloves to a food processor and pulse until they are small pieces. (Note: if you do not have a food processor, you can still make this, you just must chop the ginger, garlic, and onion until they are finely chopped into almost a paste).
Once the ginger and garlic are chopped finely, add chunks of onion to the food processor and pulse the onions along with the ginger and garlic. Keep pulsing until everything is pureed into a smooth paste. Place the pureed paste into a small bowl for use in the sauce.
Chicken seasoning: in a large bowl add all your chicken pieces cut into bite-sized, 1-inch pieces. Add garam masala, salt, black pepper, and cumin to the chicken. Using your hands mix the chicken pieces and spices until all the chicken pieces are coated with spices.
Chicken cooking: add vegetable oil and butter to a large pan over medium heat.
When the butter is melted and bubbling add chicken pieces making sure not to crowd the pan by putting the pieces too close together. Cook each piece of chicken for around 6 or 7 minutes. I like to cook them for at least 5 minutes on the first side without moving them so that the chicken will take on some browning color. Doing it this way, you end up cooking it for less on the second side, but the chicken will still be cooked fully, and it also is going back to cook in hot sauce later in the process.
Once all the chicken is cooked you can move it to a bowl to rest. The chicken will cook a bit longer in the sauce, so you don't have to worry too much about undercooking as long as you cook for 6 or 7 minutes. If your pan isn't large enough, you may have to cook these in batches.
Leave all the residual oil and chicken drippings in the pan after removing the chicken.
Tikka masala sauce: when all the chicken has been removed and is resting in a bowl, add another tablespoon of butter to the same pan you cooked the chicken in over the same medium heat. Add the ginger, garlic, and onion mixture to the pan and cook, stirring occasionally for 3 to 4 minutes or until everything is very soft.
Add tomato paste, garam masala, turmeric, cayenne pepper, and brown sugar and cook for 2 minutes, stirring frequently to cook the spice.
Add strained tomatoes and cook for 5 more minutes. At this point, you can add the cooked chicken pieces back to the pan with the sauce.
Add 1/2 cup of heavy cream, stir very well to combine, and cook for an additional 5 minutes to warm everything up.
Finish the sauce with 1 tablespoon of lemon juice and stir everything to combine. Remove the pan from the heat and get ready to build your sandwich.
Sandwich assembly: slice a six-inch roll 2/3rds of the way through and fill it with chicken and tikka masala sauce. Make sure a tablespoon or so of sauce gets into the back of the roll to ensure the sandwich is sauced enough.
Garnish the sauced chicken with diced cilantro and top it with a few French's fried onions to add crunch.
Serve and enjoy.
Notes:
I do not consider this recipe to be very spicy. It is pretty mild, but you can remove all of the cayenne powder to eliminate any spice heat that this sandwich might have.