Cut your tomatoes and jalapenos in half lengthwise.
Line a baking sheet with foil. Place the tomatoes and peppers cut side down on the foil.
Cut and peel the onion, then cut in half (two wedges). Place the onions cut side up on the foil.
Add the garlic cloves, still in their skin, to the foil pan.
Place on the top rack of the oven and broil until the skins of the peppers and tomatoes are turning dark and blistering, approximately 5-10 minutes depending on your oven. Remove and set aside.
Let the vegetables cool slightly until you can handle them. Remove the skins from the tomatoes, peppers, and garlic. Remove the seeds and white ribs from the jalapenos if desired to remove some of the pepper heat.
Place the tomatoes, peppers, garlic cloves, cilantro, lime juice, cumin, and salt in a blender or food processor. Blend until smooth.
Season to taste as desired with salt and lime. If desired, make it thinner with some water.
You can use pre-minced garlic. Just simply skip the roasting step and add to the blender/processor with all ingredients. It will not affect the flavor too much.Want it spicy? Use a hotter pepper or use more jalapeno.