Place a large heavy sauce pot or Dutch oven over medium-low heat. Add the olive oil and whole garlic cloves and cook slowly until the garlic is light golden brown, caramelized, and soft enough to mash, 7 to 10minutes.
Gently mash the garlic in the pot with a fork or the back of a spoon. Add the sliced garlic to the pot, decrease the heat to low, and cook gently until aromatic but without color, 30 seconds to a minute.
Add the tomato paste and cook, stirring, until the mixture starts to turn a deeper red and is well combined with the oil and garlic, 3 to5 minutes.
Add the Calabrian chiles, chile flakes, and fennel seed, then add the tomatoes and their juice, stir to incorporate all ingredients, and cook until the flours have blended, 30 to 45 minutes. Season with salt q.b.
Set aside off the heat until ready to use, or let cool, transfer to an airtight container, and refrigerate for up to 5 days or freeze for up to 1 month for another use.