In large bowl, place lentils, beer, and 2 cups (500 mL) water. Stir together. Cover and refrigerate for 8 hours.
Preheat oven to 375 F (190 F). Slice just enough top off garlic head to expose cloves. Place in loosely fitted bed of foil. Drizzle with 1 tsp (5 mL) avocado oil and a bit of water. Bake in oven until cloves are soft when pressed, about 40 minutes. Remove and cool. Place roasted garlic in small bowl and refrigerate until ready to use.
Once lentils have fully soaked, drain and place in large saucepan along with 3 cups (750 mL) fresh water. Bring to a boil; then reduce heat to medium-low and, with lid ajar, simmer for 20 minutes, or until lentils are tender but not mushy. Drain well and set aside to cool.
Once cooled, place lentils in food processor along with onion, cilantro, flour, cumin, coriander, salt, and pepper. Pop roasted garlic cloves from their skins and add. Pulse until a coarse meal forms, occasionally scraping down sides of food processor with spatula. Texture should be like a firm, slightly chunky cookie dough. Do not overprocess to a smooth paste.
Transfer mixture to large bowl. Cover bowl with wrap and refrigerate for about 2 hours to slightly firm.
Preheat oven to 400 F (200 C). With damp palms, shape mixture into 2 in (5 cm) balls and slightly flatten into patties. Place on parchment-lined baking sheet in a single layer.
If mixture is too loose, return to bowl and stir in a little more flour, a tablespoon at a time, just until it will hold together. Be careful, as too much flour will make them dense. Lightly brush patties with remaining 1 tsp (5 mL) avocado oil.
Bake in oven for 10 minutes per side or until golden. Serve with Smoky Tomato Sauce. Delicious as sliders with arugula and sliced avocados, it can be served as an appetizer or a main course.